not familiar enough with the specific U.S. rules about 401(k)s or withdrawals.

Unless it's a major portion of your wealth (i.e., it's much, much bigger than all your BTC or what you could expect to earn in the next few years), I would probably just keep it around as a backup.

My country forces me to pay into a 401(k)-like pension system (and gives me ~25% tax discounts for additional contributions), so while I don't max that out all the time (hashtag cashflow) I'm happy to keep it around as a bitcoin-failsafe protection. Basically: let's posit (as market prices are telling us these days) that I'm completely wrong about bitcoin. At least I won't starve in old age.