So if I use coinbase, I still have to link using my regular checking account? Is there a direct method of buying BTC directly from an online 401k account?
I'm pretty sure you can't. Your best bet is to ask the company that manages your 401k.
If they don't know what bitcoin is, then ask them how to invest in physical gold bullion coins. The answer for bitcoin will be the same unless they have a some special program for investing in gold coins,.
I know of two bitcoin ETFs being set up right now. So, in the near future you might be able to invest in one of these ETFs through your 401k.