Maybe doing less now, and then some upgrades when you're ready to sell is something to consider.

But you dont want to do what some did with Morris invest and paid in full up front in cash for a new build that never got built and they lost all their money..  5k  thats not much risk at all.. we usually get quite a bit more than that if we are going to modify any thing. and for upgrades they pay for those as they go in .. if they walk they lose it..  

My thoughts are if the siding is upgraded (see link above) the appraiser may be drawn to the higher end of the range for the comps (total gut and rehab inside so everything will be clean there).Thanks in advance for your thoughts and help everyone!!

But MAYBE you can find some expenses or upgrades that can write off the difference?  

Kitchen has upgraded appliances and granite countertops.

The owners are collecting rent below market value because the units are outdated and out of place in that market, if upgraded we can move to column 3 which reflect the value of units if updated to (granite counters, prefabricated high-grade counters, stainless steel appliances, constituted wood flooring,  split ac units instead of window units, new windows (hurricane proof) and a driveway instead of dirt) which will match the neighborhood. 

-- Would adding another unit involve having to upgrade the electrical system to meet current code?

How about creating a new category of "slow-habs", in which you do only the repairs and upgrades needed for the property to be rentable (in the currently hot market that allows higher than usual rents), then schedule the contractors for months down the road when they're available.

If the $650 properties are in the same condition and have the same level of upgrades as the subject 4plex, you can simply raise the rents without having to perform renovations.

I am the same as @Dan Maciejewski no mods without written permission but we allow holes for pictures or curtains and request they patch and touch up before leaving.We do work with people on upgrades if the upgrade increases the appeal of the property.