Kids that age are not easy to organize.
At our 4 year old birthday, which we held at our home we did the following:
Met at a playground, when everyone was there, a simple treasure hunt where they followed some balloons we tied to trees.
After everyone got the treasure, we went back to the house for snacks, cake and playing with the balloons and whatever random toys they could find. They entertained each other.
We unwrapped the presents with everyone there, the kids like to hand their present to the birthday kid!
We were at another birthday party with a similar party, but they did an indoor treasure hunt with clues, the dad read them aloud - super popular (where do you lay down at night? Clue was for the birthday girl's bed).
They also had a bunch of craft stuff, the kids could sit down and draw their own (pre cut) masks. They all loved that. I think if you have kiddie table space enough, anything crafty, like drawing is fun. If you have a theme you can use some printouts for coloring.