Someone might have already answered this, but I haven't come across a response yet. BUT .. No
making a FP between 4 & 6pm prior to official party time will not take away / use FP for another day / another park. MVMCP ticket is not considered a regular ticket, it is a party ticket; which is why you can't get in prior to 4pm with a it, and also why you can't stay after 7pm without it.
If you haven't already made FP, worry not .. once you're able to, you will be able to make them for each day of your trip, even if you make some for the short time available before MVMCP begins.