I would talk to the bank first.

They simply skip over/ignore concepts like vulnerable persons, equity skimming, equity stripping, bank fraud, wire fraud, mortgage fraud, escrow and closing, etc  It's now the "thing" to do.What could go wrong?

Sounds like the velocity banking stuff that was popular for a while.

It may create a problem for you later if your bank discovers the account is owned by an LLC that no longer exists.

It is investment banking, so the rules are much different than your "normal" lenders who offer mortgages or loans to cover a single or portfolio of collateral. 

Every loan and bank will have its associated fees and cost as its the price to do business but if you see more leverage in tapping into existing equity that could very well be the case.

What does my purchase price and down payment need to look like for the bank to provide financing? 

You can probably just go ahead and live in that 5th unit, but I believe whenever you go to refinance it with a bank or what not, the 5th unit will make it require a commercial loan and not residential regardless of what the county records show the property to be. 

At this point, I would recommend having a line of credit to cover these things so you don't have money sitting in the bank doing nothing when it could be put to work.I have 33 units, no debt except for mortgages, and excellent income.

I also liked that they are US based as I had issues with my bank with the Finland based Hostaway.Finally, I highly recommend PriceLabs.