There's a saying... When you fail to Plan - You plan to Fail.
Before, dropping the kind cash you guys are, for your aeration systems... at some point, you must ask yourself - Will it work? Am I doing it right? The first time?
The pumps you guys have, and the diffusers, will work. No doubt. Yes, 4 heads.
The restriction, as pointed out, is the airline diameter size, and length.
Imagine you load up the family truckster, and all head out for some Shamrock shakes. You walk away from the counter with the goods, and over to the condiment station, where you select Coffee size straws for everyone to get get the job done!
Judging by the shades of purple everyone is turning, you proclaim - The shakes are defective. You head over to the manager and demand satisfaction, where he tells you - "if you are dead set on using those straws, you'll need more sucking power, or, you could just get a bigger straw".
By increasing the airline from 1/2" to 3/4", your flow increases by more than Double. Yes Double. And with the same very low power draw that you are enjoying. No need to step up the horse power, and the monthly energy bill that comes with it, all because you saved a few bucks on the airline.
A larger airline is a one time cost that will be long forgotten, while the energy savings continue.
In the case of running a 1/2" airline 100 feet, then splitting it 4 ways, and expecting uniform flow across 4 diffusers. That sounds a lot like a defective Shamrock shake.
Uniform flow at multiple diffusers, is done by balancing flow at a valve control station, with multiple lines, preferably near the waters edge. I'd separate those heads (at least to doubles), I'm not sure why you'd want 4 diffusers all together anyway.
