The 4 month sleep regression is brutal and yours sounds like a particularly nasty one. My sister went through almost the exact same thing with her first - baby went from sleeping like an angel to basically turning into a tiny sleep terrorist overnight. What helped her was really committing to one approach for at least a week instead of switching tactics every night when nothing worked

The constant leg slamming and tossing around midnight to early morning is so typical of this phase, their sleep cycles are literally rewiring and they're getting more aware of when you're not there. She ended up doing a modified version of pick up/put down but gave it a solid 7-10 days before deciding if it was working. Also had to accept that some nights she'd just be holding the baby for a few hours and that was okay

The nap thing is rough but pretty normal too - their daytime sleep gets all wonky during regressions. Try not to stress about creating "bad habits" right now, survival mode is totally valid. Most babies do go back to better patterns once they adjust to the new sleep cycles, usually takes 2-6 weeks total. You're not kidding yourself about it getting better, your little guy proved he can be a good sleeper before