Don't stress, it's very easy when you only have 4 (total). The biggest problem you may face is if one of the girls isn't ready for a night away from home.
It's proably best to invite them to arrive around 5pm and you can serve dinner, ask your daughter what she wants, pizza is the usual. With just a few girls, I've taken them out for pizzza. Less mess in my house, and the girls seem to enjoy going out. If she wants something else, cook it ahead of time.
Ask your daughter what type of activities she would like to do. If a movie is planned, pick out 2 ahead of time. If you give them a large choice, it'll take them forever to decide and on one will agree. Don't forget about the popcorn.
Having music available is always a good idea. Encourage them to do silly dances or teach them one from when you were growing up.
You really don't need to spend a lot of time with them as they will keep themselves occupied. You may want to consider one activity, maybe manicures or a craft project.
Before lights out, make sure they know where an extra blanket is if they get cold, make sure there's a night light(s) so they can see the way to the bathroom. Let them know if there's a problem they can come get you, or have your daughter get you. Make sure they have access to water if they get thirsty, you can give them water bottles or leave cups in the bathroom.
Make sure you have a list of emergency phone numbers so you can get ahold of the parents.
Plan a simple breakfast for the morning. Our favorite, bagels w/cream cheese, some fruit and some yogurt and juice.
Let the parents know what time to pick them up. Arround 10:00 is good.
Make sure you schedule some quiet time or nap time for yourself the following day.
Have fun!!!