What does your daughter like?
Does she have a particular film or show she's mad about, a colour or animal etc you could then use that as your decorating, cake, party bag theme.
At 4, DD was in love with all things unicorn and rainbow, so would have been thrilled with lots of sparkly stuff.
In terms of activities boy or girl they're pretty much all the same at that age and a bouncy castle, shit tonne of balloons and area where they can dance to the Trolls and Disney soundtracks will make pretty much any 4yr old happy.
Food, I always keep it simple and stick to a vegetarian menu. They aren't that interested in the food, just want to eat enough to re energise for more bouncing, running and screaming.
Cheese or Jam sandwiches
Crisps
Cherry tomatoes and mini cucumber
Melon slices and berries
Selection of fun biscuits - chocolate fingers, jammy dodgers, party rings
Selection of mini cakes - mini rolls, mr Kipling, fairy cakes
Breadsticks/cheese straws
2 hr party is my max limit, after that they start getting worn out and mardy. 1st hour for above mentioned bouncing/running/screaming, then 30 mins for food and blowing out the candles on the cake, last 30 mins they do more running/bouncing/screaming while you clear up the food, cut the cake for party bags and everyone starts filtering away. Send them all home with a couple of balloons each as well to help reduce that tidy up.
For party bags I liked the £1 colour in tote bags from The Works, then I'd use that to hold a slice of cake, pack of haribo and some kind of little trinket like a pot of slime/putty, pencil and eraser, temporary tattoos, sticks...something along those lines.