Once you have used your 3 pre-booked, you have to go to a kiosk to book your 4th. After you have used your 4th, you have to go to a kiosk to book a 5th, etc.
If you use your 3 in one park-- when you travel to a new park you can go to a kiosk in the NEW park and book your 4th there. You must be at the new park to book a FP there.
Based on our FP+ trips, as far as booking a 4th FP+, MK can be useful- the rest of the parks not so much. I have tried loading our FP+ at DHS early in hopes of getting a 4th FP+ for something useful there and have been entirely unsuccessful. We are typically done with our FP+ by just after 11:00 and every time I have tried I have been advised at the kiosk there were no FP+ available for the rest of the day- not just nothing good-but none at all. We have tried loading our FP+ early at EPCOT in hopes of picking up one for either TT or Soarin, whichever we did not choose as our Tier 1 that day-- we have never been successful. We have never been able to pick up either as a 4th. We have been able to pick up Figment and Spaceship Earth- and before you laugh about them not needed- I picked on up for Figment in the afternoon because the Figment line was 30 minutes long - and it was not a day the park was overly crowded either. MS- if you are riding the green side, we rarely need a FP+. AK- there probably aren't even 3 you need in the first place, but I wouldn't plan on going over there and picking up a 4th for Soarin or EE, as those are the 2 primary ones that people book there.
