Need advice on water heaters for 4-unit building

[Daniel Haggar]

I have recently purchased a 5-plex. 4 of the units (420ft efficiency) are in the same building. The units have no washer/dryer or dishwasher. Very minimal stuff in the units and they will be single occupancy. There is a crawl space where all utilities are separate (gas/electric/furnaces/water heaters). The crawl space is a pain in the butt to get into and water heaters are not easy to acccess. The previous slum-lord left 13 water heaters behind in the crawl space that I removed!! 

There are currently 3 water heaters installed and one of them is servicing 2 units (previous owner did this after one died and he didnt want to replace it and the tenants didnt know). I need to replace 2/3 of the current water heaters. I'm wondering if there is a way I can install another water heater and have 2 heaters servicing all 4 units. The reason for this is mostly for maintenance and efficiency. I feel it is very inefficient to have 4 water heaters down there. Is there a way I can do this in some sort of a fair way for the tenants, or is it best to just haul in/hook up 3 water heaters? Any suggestions?