Oldest got one at 4. WHAT A PAIN.
She needed alot of help and we learned more about catching Pokemon than we ever wanted to! At the sitters, she had daily assistance from the local 7 year old boys. She loved the damn gameboy, and wanted to play at home too, and on weekends. Alas, though reportedly pathetic compared to 7 year old boys, we were enlisted to help her with her game.
The current 4 year old also has one. Big difference. Why would he bother us for help with our pathetic skill level, when there is a perfectly qualified 7 year old sibbling with 3 years experience available! Heh heh heh.
Two things. First, the kids don't play with the things continuously. They go through phases. In between phases, you wonder WHY you bought the things, but then they come back to them. Second, I bet your 4 year old will LOVE it, and you better LOVE it to, because you are going to be the one playing it FOR him for awhile - unless there is a skilled older sib in the family. Otherwise, I guarantee you, it will keep not only the child, but also one parent very well occupied on the flight. In the end, you'll probably get one for him eventually, so why not now. The extra thrill will add to his trip experience.