yes, this definetly does not sound like a seized caliper, as you'd have 3 other wheels which still had brakes and, if anything, the pedal would go down LESS as less fluid goes through the system (Since that one caliper won't take any).

You have air in your lines (maybe low fluid), a leak in your system, or a bad master cylinder. Almost ANYTHING else brake-related and you'd still have SOME pedal and stopping power (as in >5%)! I'd say one of your lines is probably cracked or split, causing fluid to seep out at the previous brake usage, and air to fill in its spot when you let off the pedal, causing such poor stopping power. If you pushed the pedal that hard and your rear wheels didn't lock up, I'd suspect a hose back there leaking somewhere.