Once you move into the additional passes after your original three, tiering doesn't apply. Tier 1 attractions are less likely to be available in many cases, but if you can find them, you can get as many as time allows.
after you use your first 3 then anything is fair game including tier 1 and tier 2.
however many times by the time you use your 3rd FP+ the tier 1 attractions have limited to no availability.
Taking the answer one step further, not only can you get a 4th FP+ that is a tier 1, it can be for the SAME tier 1 attraction as you rode earlier. All subject to availability.
Many details, logistics, and rules of FP+ found HERE.