I can explain why I chose #4.
One of the best parts of FP+, IMO, is your ability to change most of your attractions on the day of. So if 60 days out you make your Space Mountain reservation for 2:00 PM, but at 1:00pm that day your family is still enjoying the afternoon at the pool or in another park, currently you can change your Space MT fast pass for later that afternoon or evening.
IF you allow people to make 4th, 5th, 6th FPs once they complete either one FP or all three FP, you're going to have people taking additional FPs by 12:00pm. This will severely limit anyone else's ability to change their original FPs for later in the day. Clearly those people will use their 4-6th FPs to reserve the headliners and you will be stuck in your reserved spot.
Personally, I like the 3 FPs knowing there WILL be additional times to move them to if needed.
Wasn't everyone complaining that FP+ forced them to plan their days down to the minute 60 days in advance?! Well, if the rope droppers are making their 6th FP reservation before you've used your first FP, it isn't much different than the FP- system, is it....