If you have an 401K that is not managed by your employer (I had to leave the company that held my 401K before I could do this)

You can roll that 401K into a regular IRA and then open an account with unchained to buy self custody btc.

You can roll it into a roth as well, but there is a tax liability.... looking back it might have been wise for me to pay the tax and make it a roth.