Like several posts above, i too express great caution in mixing puffers. All situations are differant. Like above tank size matters, the type of puffers matter, how you introduce them, They need to be as close in size as pos. if you chose to go that way, I'd get a larger tank for it. Just like some people think figure 8 puffers are brackish water, and some think u should add more salt slowly as they grow untill they are at full salt. You will have to, in the end, trust your judgment on how well you know behavioral patterns. If you know that and give each fish room and research compatibility. You are more likely to have sucesses .
My puffer is alone and has twenty g. just for him and i don't want to risk anything untill i get a larger puffer tank and will have the same kind more then likely. love porcupine puffers and fig.8. Lots of people do have luck in getting puffers that are nicer then most, though im sure they can defend there self. As they age and mature they often start to define territories and if enough space is not provided, they will start to fight over the given space.
Alone But Not Lonely
The link might help more then me, but remember that like all dogs act differant so do puffers. the main reason puffers nip is thought to be lessended by giving them a large amount of snails. make sure they have something to peck at then each other. I breed ram-horn and mystery snail. the ram snails breed at fast rate and I have a tank just for them.
Good luck
p.s.
"I would not put 4 in a 90 gallon. maybe more like 150. " I agree