Gotta do what makes sense for your own situation but I'll give you some insights into my own plan. With a 401k you can typically take the greater of: $10k or 50% of your 401k balance as a loan up to a maximum of $50k.
The interest you pay is going back to yourself, at a low APR, and you're now using pre-tax money to help with repairs, a down payment, etc.
My current properties have all been financed post-tax but eventually I plan to buy a property using a loan from my 401k + conventional financing. I.e. buy a $60k cashflowing property in the midwets using a 25% down loan, downpayment is covered by a $15k loan from my 401k. No hit to my post-tax savings, all of the money will eventually go back into my 401k, but now I gain another cashflowing property.
Look into the exact details of your plan to see repayment period, loan terms, etc.
One thing that is pretty standard with a 401k loan is that if you leave your company (fired, laid off, quit, doesn't matter) then you will need to repay the loan within a set period of time - typically 60 days. That's an important caveat to keep in mind with this strategy and you should have some liquid form of investments such as stocks that you can tap into should you need to repay these funds.
