Hey John, I'm looking to use a 401k loan to get going as well. Based on my research and conversations with others, here's my two cents.
First, make sure to use a loan and not a disbursement. You probably already know this, but with a disbursement, you'll be hit with taxes and a 10% penalty. But with a loan, as long as your plan allows it, there are no taxes or penalties as long as you pay it back.
Second, if you are going to use the BRRRR method, then it can make a lot of sense because you are acting as your own hard money lender. Basically, you'll just be pulling the money out of your 401k for a few months and paying yourself 6.5% interest. Whereas if you use hard money, then you'll pay someone else 10-12% plus a few thousand dollars in points and fees. So if you believe that you can successfully execute a BRRRR transaction and pay the loan back within a few months, then why not pay yourself the interest and save on the thousands more that a hard money lender costs!
Last, if you are not planning to refinance the property, then I'd be very hesitant to use a 401k loan. In this situation, your monthly 401k contributions are going to pay back the loan rather than increase the size of your 401k. And if you are missing out on adding principal to your 401k for years, then you are risking your retirement.
And just in case you haven't come across this info yet, you can only borrow up to $50k or %50 of your 401k, whichever is less.
I hope that helps and good luck!