I don't mind the current Generations Voyager Grimlock, it does take a bit of manhandling to get the tail together and organising the legs into place while not difficult can be a bit tricky from a spacial perspective. The light up gimmick is pretty cool for a kid though.

the older classics grimlock is a little complex. The legs on the beast mode are a bit more articulate.

Apart from them I don't have a lot of Grimlock experience. I'm not sure any of them would be particularly easy for a 4 year old to transform. They would both be playable by a 4yo with assistance to get between modes.