Imagine: Next year, America creates a Tetrarchy, with four presidents, each with their own region inside the United States to rule. Who are the four leaders, and where do they rule? Does this work?

Texas secedes but after the first year's staggering budget shortages begins raids into Louisiana and Oklahoma with demands for the Federal tax revenue lost during secession to be resumed. No raids into Mexico because the Texas National Guard are too scared.

megacity states
Northeast Corridor aka "The City" (capital NYC) - Leader: Adam Friedland, Mayor
Great Lakes (capital Chicago) - Leader: "The Committee"
Delta-Piedmont (capital Atlanta) - Leader: Mike Smith, former Atlanta Falcons Coach and voted NFL Coach of the Year 2008, 2010 and 2012.
NorCal (capital SF) - Leader: an LLM with Taylor Swift's face that appears on all screens within 5G range of Alcatraz.
each claims far more territory than they can effectively project power over. temporary alliances to share maintenance and security responsibilities over rail transport between them are rare.
caveats
cat5+ hurricane followed by persistent wet bulb event reduces population of Florida by 69% in 6 months. disease is rampant. political structure of survivors unknown. many fires are seen but those who approach are fired upon.
Texas attempts succession and forms New Republic of Texas, immediately steeped in ultraviolent sectarian holy war between First Baptists and Southern Baptists with chemical and biological weapons deployed by both sides immediately, annihilating 90% of the population.
the rest of the former US not immediately subjected to violence of the city states over their surroundings and their marauding, demobilized ex-national guard forces degrades into manoralism and occasional warlordism fueled by remote caches of national guard armories. best described as a shifting mosaic of heaven and hell.
- I like this idea, it's a little like ancient Greece.
  I feel like Atlanta becomes Sparta, creating a military-sport warrior culture which nonetheless fucking sucks at both fighting and sport and relies on apartheid slavery to do literally anything.
  NorCal, so far from the others and in an increasingly desertifying climate due to climate change must adapt or die and succeeds due to their resources, recreating the Islamic Golden Age in this analogy and is the first to reach fusion technology of the four (I'm assuming the rest of the world has decided to go no-contact after a little bit, realizing that they will be too busy fighting between themselves to do imperialism on the rest of the world). Silicon Valley continues down the path of trying to hack the human psyche to maximise profit and ends up creating its own religion.
- political structure of survivors unknown. many fires are seen but those who approach are fired upon.
  yeah that sounds about right, I hate it here

I'm not sure who the leaders are, but I know if you defeat all four, you get to fight the secret boss.
- it's henry kissinger in his eldritch form
- you must defeat Godtrump the Grafted; Biden, Lord of Malarkey; Wokescourge Ron; Warren, Queen of Plans; Alexandria, Blade of Bernie; and the accursed brothers Musk and Zuckerberg.
  then walk into the gigantic Amazon warehouse-office skyscraper visible all across America to face down Jeff Bezos, the final boss of capitalism, before he himself is wielded as a greatsword by the hand of god itself, the Free Market, which takes the form of this colossal, grotesque, pulsating mass of flesh which represents the combined weight of its victims over the last 250 years or so.

De Santis leads the South east of Mississippi river with an iron fist. There are roaming death squads capturing LGBT folks that frequently venture outside of their borders. Yes, it does work. Businesses are stable and profit margins are higher. Half the population is homeless.
- Would lose to the Amy-Targaryen-Klobuchar Midwestern imperium tho.
- Businesses are stable and profit margins are higher
  American business owners breathe a sigh of relief, Malthusianism takes hold and tens of millions of people die. With real-life productivity going down because of all the death, the capitalists go into the Metaverse and engineer it such that it simulates the perfect capitalist, homo economicus society which generates fake profit which is nonetheless treated as real profit. The people inside that simulation then create their own, even faker and worse quality simulation, and so on, and neoliberal economists laugh at those who said that capitalism's assumption of infinite growth makes it destined to fall. This keeps going until 2135, when the population of America is down to about a thousand people. The last power plant shuts down due to lack of maintenance and all the simulations (at this point we at 45,423 simulations deep) blink out of existence in an instant. The survivor capitalists step out of the bunker and have to start civilization from scratch.

Historically the Tetrarchy featured 2 senior emperors and 2 junior emperors. I don't want to speculate over who would wind up on top after America deals with it's Crisis of the 21st Century but I will lay out the likely regions.
The Senior Blue Tetrarch rules from Washington DC and has direct rule over the region of The North as well as the overseas colonies (Hawaii, Guam, Puerto Rico) and Alaska. The Junior Blue Tetrarch rules in California and has control over that region (Washington, Oregon, Etc).
The Great Lakes region is broken up into semi-independent city-states ruled over by the Water Barons. They largely pledge fealty to the Senior Blue Tetrarch and provide him with tribute and Condottiero. They in turn extract tribute and Foederati from the northern part of the Midwest.
The Senior Red Tetrarch rules from Texas, which is the only place in the Empire that has a nuclear weapons plant. He also has a great deal of indirect control over both the Southwest and the more southern portions of the Midwest, which is also where a great deal of the empire's Foederati come from. The Junior Red Tetrarch would rule The South.
Florida remains a confusing basketcase of shifting ideological loyalties. It's not a question of who controls Florida but who controls which parts of Florida right now.

Newsom - Everything west of the Rockies.
Eric Adams - Everything east of the Alleghenies.
Pritzker - Central US above the Mason-Dixon
Abbott - Central US below the Mason-Dixon
- This one's the most cursed
- This, but instead of Newsom, make the western nation a Chinese protectorate led by President John Cena.

Well, you said "America", so... In the north, will be the province of Canada, ruled by Justin Trudeau. South of that, a lawless land under the grip of a certain Donald. Then the province of Mexico, under Claudia Padro. And, finally, Central American Region under the leadership of Miguel Díaz-Canel.
- Central American Region under the leadership of Miguel Díaz-Canel.

Currently, the most likely scenario is that Texas leaves and brings Florida with it, with the second most plausible being that California, Oregon, and Washington leave as a group. They're both mutually exclusive scenarios, but neither of these are currently particularly likely as both of the groups behind each movement are basically competing to have their side control the federal government and be the voice of the entire nation.
The Deseret Nationalist weirdoes are deeply unpopular and any sincere aspects of the modern Deseret movement want to remain part of the US and mostly just want to change Utah's name and expand its borders into surrounding states to encompass more Mormon communities currently in those states.
Neo-Confederates are a joke, they're perfectly happy just taking over the US government, there are legitimately like under 30 guys in League of the South that actually still want to secede.
Alaska's independence movement is basically just their Libertarian Party and was the first to be fully coopted back into the GOP, like 15 years before the Tea Party movement even started.
The New England independence people are a handful of online goofs who want to unify three different movements that are mostly defunct. Their entire existence is one website, and the goals of each of the movements in NH, VE, and ME are in direct opposition to one another so tying together for viability will be impossible. Each of those movements is basically dead and amount to a website and a slogan, a commune in the woods with no recruiting or goals, and a facebook group, respectively.
The Chicano Movement has largely given up on nationalist aspects like the concept of forming Aztlán as an independent nation. This is due to a bunch of different factors, but COINTELPRO was no small part in disempowering and redirecting that energy during the 70s.
The Republic of New Afrika has effectively ended as a concept thanks to COINTELPRO. Any active org that claims that as a goal is at best populated by Feds for the purposes of monitoring leftists and at worst an Op to make the concept unpopular.
Various Indigenous movements currently exist, but are actively being suppressed by both the government and settlers on the land. Proposals like the Republic of Lakotah don't seem to be likely to gain recognition, unfortunately.