At that age for my daughters' parties we just pretty much let the kids play with all the toys at the house. You could set up a playdoh station, a coloring station, and then a free play station. The kids will entertain themselves for the most part.
I know it doesn't sound too different or creative, but my experience was that any of the formal activities I tried were "OK", but mostly the kids just played with whatever was available and had fun that way. I also had to deal with a range of ages like you, and 1-4 is actually quite an age range when you consider their abilitites at those ages. Finding an activity that they can all do together is challenging.
Maybe they could decorate their own cookies or cupcakes? Even the young ones could do that although the clean up would be more perhaps.
You can also check out orientaltradingcompany.com, they offer party supplies and craft stuff at pretty reasonable prices.
Have fun planning your party!
J.