I've been trying to juggle some upcoming stays between Marriott, Hyatt and Hilton, and the new cat changes have made the decision for me. After May, the Cat 4 certs are going to be useless to me as I rarely have airport or suburban stays.
Well if you earn them now, and then make the ressies if you know the date, then you can book the hotel now in the existing category, as long as you make the ressie before the cat changes, then you are fine.
Update: Mr. Nacho had 2x Cat 5 cert promo, and since he is running Plat Challenge he will have a pile of stays, so we asked to swap his promo to 3x Cat 4 and it was granted.