I honestly think it's best to forget about the possibility of a 4th FP. I MIGHT make an exception for MK, but even then I think it's more valuable to have 3 headliners booked for mid-afternoon or evening rather than using them in the morning getting very little benefit from them in hopes of getting a 9 pm Space Mountain FP or several afternoon FPs of rides that only have about a 20 minute wait anyway.
You spend 5 of those minutes walking to a kiosk and waiting in line and booking them when you could have been standing in the standby line. And then your FP wait is still 5 minutes. So sure, maybe you saved 5-10 minutes on 4 or 5 rides. So say you saved 50 minutes because you got 5 additional FPs.
But your first 3 were at 9-10, 10-11, and 11-12. You probably saved no time with the first FP, the second one at best you saved 30 minutes, and the last one maybe you saved 45. (Yes, every park has an exception to this but MK is the only one that has 2.) So all in all, with your 8 FPs, you saved a little over 2 hours. Book those same 3 FPs for 1-4 and save conservatively 30 minutes each, plus getting just as much done during rope drop as the person who made FPs during that time. So for all that extra trouble you went through booking extra FPs, you saved maybe 45 minutes.