Like others have already suggested, keep the existing ones and refresh to see if you can get better times. People’s plans change all the time, and there is a good chance you may score times that suit you better. As for 4th Fastpasses, we’ve pretty much always managed to get what we want for a party of 3. We’ve never tried for FoP but have done things like Mine Train, Frozen, Soarin’ and Test Track multiple times. In fact, I never book Fastpasses for Soarin’ and Test Track in advance - I just grab them as 4th as soon as I can!
Good luck and happy refreshing!