Backyard Aquaponics
A place to discuss aquaponics

Seperate systems would be better, in my humble opinion which may very well be wrong mind you, as it will firstly allow you to grow different sizes and types of fish, so you can harvest them all year long, if there is a problem with disease in one tank the other systems will be safe, multiple systems will afford you the luxury of experimenting with the efficiency of the system, as its a teaching oppurtunity four different groups of students may be put in charge of it, after starting one up it can be used to kick start the other systems, a larger system is a lot harder and longer to cycle so it will save your setup time.

thats what I can think of off the top of my head, hope it helps

i would do four for all the reasons abdul mentioned.
lowers risk
not all your eggs in one basket
just me thinking