You can do a direct transfer rollover where your current custodian sends a check to the new custodian to avoid taxes. If you cash out and they send you a check, they will withhold taxes and you have to put the entire amount (including the taxes they withheld) into the new account to avoid paying taxes on it.

Contact whoever you're going to use for the bitcoin IRA and tell them you want to roll your 401k into it. They should be able to give you the correct form to give to your 401k custodian.