Perpetual + sourcing outside clones == recipe for disaster or poison. When (not if) you get an outbreak of something from a clone, you'll either have to tear out everything infected (usually means dumping everything in the perpetual flower) OR use some kind of organic or inorganic poisons.
4 plant limit is definitely a limit. Any way you could work your way up to 4 cabs, one cab for each plant? Hepa filtered intakes and outtakes (made from furnace filters) will keep infections/pollen separated.