I remember I asked about this on the FP+ thread for my March trip and it 100% worked. I booked Little Mermaid at HS for the first show and skipped it. Booked TSMM, then ToT and picked up another ToT while waiting in line for my first ride. Worked like a charm. The Little Mermaid FP+ did remain in MDE the entire day and I did not cancel it because then I would have still been in my original 3.
It will be interesting if this is closed with the upcoming changes. I have a HS and Epcot day planned like this for July. lol
Interesting. So even though it was still showing in your MDE, it didn't prevent you from selecting a 4th? Si it's there.. but not really.