Furnace replacement, one or many ??

[John Clark]

I'm looking at bringing an old 4 unit back to life. (2 up, 2 down) The 30 year old forced air furnace in the basement is dead and needs to be replaced. The question is, should I replace it with another single large furnace or 4 separate units?
 The good thing with 4 separate furnaces is that I could pass the heating cost to the individual tenant's. 
 The down side is that 4 separate furnaces most likely will cost more for purchase and install. 

Thoughts??