Tray jigsaws of Flying Scotsman and Mallard.
These proved very popular with our four year old grandson particularly as he was familiar with Flying Scotsman and Mallard. The number of pieces and their size was just right for someone of his age. The bright colours make them attractive to the eye as well.
He would be even more interested in diesel jigsaw especially a Deltic which he is fascinated by though occasionally the appearance of one on a railway dvd can result in a move to the sitting room door to watch it through the gap between it and the door.