For craft ideas check out familyfun.com (I think that's it, otherwise just google family fun magazine) and also look at orientaltradingcompany.com. You can actually order kits from Oriental Trading that are cheap, cute and easy to make.
I'm sure you have some old rubbermaid containers around the house and even if they're being used, maybe you can empty them temporarily...fill them up with about a bottle of Joy liquid soap (for dishes) and water. I use the ones that are about 2 feet long and about 6-8 deep. It will make awesome bubbles!! Use found objects around the house to fill the containers and let the kids go bubble crazy! I do this for every kid party at my house and the kids love it! You can use hangers, slotted spoons, 6-pack soda holders, etc. I also save plastic lids and containers to things and just cut holes in them or cut the bottoms off plastic soda bottles, etc. It's way cheap!
Other than that, i would just get out (or borrow) as many outdoor, ride-ons, balls, sidewalk chalk, etc. and let the kids go crazy. At 4 they're not going to do too many organized activities.