Help from Sleep Routine Moms, please!

We've been doing a successful nighttime sleep routine (massage, book, diaper, snuggle) with my son, but it seems like it's working so well, that he now knows what to expect, and has started crying because he doesn't want to go down for the night. He's 4 months old, and sleeping long stretches like a champ, but suddenly he's started doing a full blown sad face (complete with quivering lip) then screaming for 5 minutes straight, once he knows we've started the routine. When he stops crying, we put him down drowsy, then he's fast asleep for the night. It's like he's created his own routine, which includes screaming lol.

We thought maybe he was overtired, since he's dropped a nap. But he cries even when going down at 6:30pm. We know he's sleepy, so it's not too early. He loves his crib, so it's not that either, as he's sleeping well, and seems to be able to self soothe in the middle of the night. Naps also aren't a problem.

Any advice? I don't want him to associate the sleep routine with negativity, since it should be calming. Should I switch it up, stop the routine all together, is this just sleep regression stuff that will pass?