Your child can pick the theme of her party, bearing in mind that most four year-olds are totally satisfied with a themed cake, plates, and party hats, so it’s really not necessary to go completely overboard here.
You might want to invite the whole preschool class, but this would really be chaotic and exhausting. Let your child choose a few of her closest friends from her class - not only will this be easier for you to handle, but children usually have more fun in smaller groups as well.
Don’t let the party drag on for hours - the children will become bored. It is best to limit the party to about two hours. Make a mental note to yourself that the party is for little children. They will be more than happy with a birthday cake, plenty of playtime with their friends, and possibly a little special treat to take home with them.