S.S.
Hi! Our daughters sound quite similar...as do our birthday party efforts. I understand where you are: trying to keep it reasonable but somehow going over-board. We did a pony birthday recently. Rather then haul in the ponies, we spent FAR less (which surprised me) by going to toys-r-us and buying those " stuffed horse head on a stick" toys....you know what I mean? We were able to find boy ones and girl ones...each was about $8 or so. We went to the dollar story and bought bandanas and pink/blue cowboy hats too. The kids had a blast with the ponies...we did a lot of fun races and games with them. We also had a toy "horseshoes" game and did the pony pinata as well. The thing about 4 year-olds is that they will have fun with a box if you give it to them. They don't NEED the bounce house and build-a-bear. They will have more fun creating their own games and just running around together. If it ends up being an all girl party, we also hauled out all my daughters hair bows, etc. and let them go to town grooming the amazing amount of mane that these ponies come with. I'm not sure boys would get such a kick out of that though....
Anyway, of course we sent everyone home with their horse, hat and bandana...great in place of a gift bag! The party ended up costing far less then I had originally expected but the kids appreciated the take home items and time together far more. Everyone won!
Good luck!