You shall avoid 4th fleet when you are very small, and when you are very big.
When you are very small, because each new fleet group is a pain to pay.
When you are very big because the communality costs reach insane levels.
Whatever happens, be sure not to have fleet groups with a very limited number of planes. In the same GW3, I was tempted to take a few YAK42, but there are only 6 available on the market, and they are not built anymore, and it would cost me too much. I did let the expansion opportunity pass. If there had been 20 of them on the market, I might have taken the risk. Instead, I'll wait for my Ejets to arrive, in 2 years.
In other words, if you go for one more fleet group, go massive. It's not worth it otherwise. And be sure to have at least 10% margin before playing with the devil. If you're weak at 3-5%, better improve your current fleet utilization than to go to another type.