It's an old mower that was given to me. The cap says unleaded gas,
the manual says four stroke, so I didn't use premix.

Tank was dry, I put in gas, when I yank the starter cord it's like I'm
pulling against compression and gas squirts out the exhaust. Lots of
it.

Sounds like it's dead and not revivable, but any idea what exactly is
broken?

I admit, I don't know much about the insides of these things.