Scale back. She's 4. Parents get so crazy about spending a fortune on bounce houses and ponies and so on. Go back to basics. Kids just like to have fun. Don't have a zillion people - and as you go forward, please fight the urge to "invite the whole class" - it gets overwhelming and the birthday child gets lost in the shuffle.
Go simple. Go traditional. Put out the sprinkler and let them run through it. Run a few old style games like sack races (everyone has an old pillowcase or two) or potato races (get some cheap wooden spoons at the dollar store). Go on line or get a book at the library with the "rules" and ideas. Have an egg toss or water balloon toss - everyone can participate (littler kids can stand closer together). Everyone rinses off with the hose and sprinkler. Other adults can help supervise.
A few of my neighbors have gone overboard (they had an animal guy with 2 llamas, 2 goats, a rabbit and some chickens, plus portable pens). Others had bounce houses and 30 kids - no one had a chance to bounce due to long lines, and there were lots of injuries.
Another family did sack races and 3 legged races, then they did a craft with little foam or wooden frames from the craft store, plus foam stick-on shapes (for the foam frame) and macaroni shapes (put on the wood frames, then coated with spray paint by the adults. Someone took a picture of each kid or family and emailed it after the event - everyone went home with a frame.
We always did treasure hunts - bought a bunch of those plastic eggs and put a few small candies or toys or stickers in each one. We hid them in the yard, in flower pots, in notches of short trees, etc. Each child got a "color" to search for - we put a sticker or a piece of paper colored with a crayon on a plastic collection bag. Everyone went to find their own color - that let the little kids have a chance against the big kids who were faster and taller and better with colors. The eggs became their prizes, so no goody bags in addition. Kids even cooperated to help everyone find their own egg - so it wasn't competitive or cut-throat, just fun. We kept our parties small - none of this "whole class" bit. So everything was affordable, and the birthday child was the true center of the celebration instead of just one more body in the midst of chaos.
Guess which party guests had the most fun?