While this is a good problem to have, I'm starting to stress about it for a few reasons I'll put below.

Without knowing all of your specifics, I'd say that it's only worth holding on to if you can reasonably expect to have some really good appreciation in the near future (like 5 yrs)....Or, in the case where it would run well as a vacation rental, you could switch the usage, maybe?

If you intended to flip but now want to hold, that's one of the major reasons DSCR loans exist.

The reason I say this is that with a 5+ unit a lender will evaluate based on NOI and a cap rate, so adding a 5th unit COULD (or could not) substantially increase the value of the building and make it worthwhile to refinance and pay off the existing loan (that is tied to the "old" quad) anyway.

It’s one of the reasons I like this market—there’s flexibility to pivot between STR and LTR depending on what works best

There is a reason why I buy properties that cost a multiple of what typical cheap rental properties are going for.

The reason I say that is because, the IRS requires distributions (RMDs) in order to have the taxes get paid on that money eventuallyl. 

I've been reasonably happy with this so far, since it's made the process fairly passive. 

If the LLC is a single-member LLC, it is considered a disregarded entity for tax purposes, meaning the taxpayer remains the same (under the same Social Security number) and there would be no reason to change title prior to the sale.

Even this is an incomplete list.Worst of all, the main reason not to make this loan is that you will not hold a first position-lien against the property.