Hi there!! My step son is also 4 and he went through a phase where he was being a stinker about being in pictures. The more attention you give him about it and the more you beg him to "please smile" the more attention he is getting and the more he will act up. It's become an attention seeking situation and it seems to be working!! I'll tell you what worked with for us...
this is only my opinion of course...(little disclaimer!)
Seeing as how TJ is only four, I do understand being silly and not wanting to stand still, but really we aren't asking all that much of him. When comes down to it, he is being given a direction to stand still and smile for a couple of pictures. He can totally handle that. If he refuses and hides his face or frowns at the camera, he is being defiant and he gets a consequence for it. He is promptly removed from the picture, placed in time out and is not given another second of attention for inappropriate behavior. Pictures keep going, his sister is praised for doing a good job etc. When it came time for more pics, if he continuted to act up, same situation. Removed from the action and bad behavior ignored. It wasn't long before TJ wanted to show us how big HIS smile was. When he realized that acting up was only going to result in a consequence and being left out of the fun he changed his attitude promptly. When his dad and I got married this August he posed and smiled his four year old head off. He did GREAT!!! Bottom line is he needs to do as he is asked. It's not his choice to ruin pictures. Asking him to pose for 10 pictures in a row would be too much, but just a few is totally acceptable. He needs to follow directions cuz at 4, that is his job. Anything else is not okay.
I just have to say in sort of a tangent, that I was APPALLED at some of the behaviors I saw at WDW...kicking screaming fits, kids hitting their parents, talking back, saying no...the whole gamut of misbehavior. And it always seemed to be the kids who had a balloon and stuffed Mickeys and candy and every other thing their little heart desired. It's not fun for the parents or the kids when they act that way and letting them act that way is doing them a total disservice...but then that is the social worker in me. Anyhow...just my two cents!