As someone who travels out of PHX T4 regularly, I think 1.5 hours is not a wise idea.

After clearing immigration, you are fed back into the landside area of T4 and then have to re-enter security. You also mentioned US First, which will help somewhat, but US and WN are the dominate airlines in T4 and there are times during the day when the back up for screening can be substantial, even with using the First lane for priority screening.

Adding another variable: if your US flight is not on the same concourse, the walk (even using the moving sidewalks) is substantial if you are arriving at the B international gates and then have to catch a flight leaving from the far side of the A concourse. (Here is the map of T4 - PDF warning.)

Safe travels.