My 4 y/o loves tinker toys, if you kid loves to build I am sure he will like this toy. His favorite gift form his party was a marble run thing. You build a track for the marbles to run down and he loves to build and change and improve on his creations. Blocks are of course great if you don't already have a set, and there are so many different kinds.
My nieces and DS female friends seem to really like the tea set toys but some more than others. Is your daughter into playing house? If so she will probably like it alot, if not, than not so much.
Pirate ships seem to be very popular with this age. My mom has taught 4 yr olds for over 25 yrs and bought one for my son based on her experience with what was popular in her class, however the one she bought (that was in her class) is a pain to put together so we have not done so yet. One of DS friends has it and his parents say it is a favorite toy. I have noticed all the kids in his class like to play pirate, girls and boys.
Two wheeler bikes or balance/glider bikes are a popular gift at this age.
eta - Woops, I posted as I was leaving work and though you were looking for your own kids, not for a party. Some of my suggestions are hmm, a bit overboard for a b-day party gift. Here are a few other suggestions:
Games: Kids on Stage, Caraboo, Balloon Lagoon, Sequence for kids, Zingo, (some may be a bit expensive but it could be a combo game since they need to play with another person anyway)
jewerly making kits
Keyhead fire station (at least my son liked this one, though it is not the best made toy ever)
Matchbox racing cars things
potholder weaving kits
Bethoveens wig CDS - (classical music set to silly words, DS loves it and when ever we bring it to school it is a big hit.)