Hi Stan,
My advice would be that you can either put in the standard number of extinguishers, or put in the amount you would feel comfortable with.
At the end of the day, if the legal requirement was one extinguisher per floor (I have no idea what the requirement is, by the way), would you feel happeir if you put one one each staircase too.
If the answer is yes, then perhaps you should do that. It's piece of mind from a personal safetly point of view (rather than a "oh shit, my house burned down, what's my insurance like", even if a tenant has died). Does that make sense?
Also look at your fire escapes if it's a 4 story building. If these are good enough, you can arguably make a compromise with with extiguishers?
Mark