your entry wont consume a regular park day ticket if you enter after 4.. but if I remember correctly(someone correct me if I am wrong), but you can book 1 extra day worth of Fastpasses with your ticket loaded to your account. I.e. you buy a 4 day, you can book 5 days worth of fastpasses(give opportunity to plan to move a park day in event of weather). So if you are using the other 5 days I'm 90% sure you can book fast passes for 6 days, or if you have a 6th day that you won't use then yes Book away on that day.. but remember you will only make your times after the party ticket entry of 4..
Last year we did VMCP and iirc had a pirates from 3:20-4:20(we got in at 4 and just walked over to pirates before it ended), HM at like 4:45-5:45 and Space from 5:50-6:50.. so we were able to knock out the rides our family wanted before the party started. There are no FP during the events(only ride really where you may need it is Seven Dwarves if you go to an earlier in the season party). But being able to basically have a quick 3 hour FP/stand by ride period, followed by a very relaxed party made the experience great, I highly recommend consuming a FP or 2, or 3 if lucky on party days between 4 and 7 to optimize your day.
