My grandson's birthday party.

you seem to be on a lot of threads CW

You could do something like a pirate party as a theme- easy to improvise a costume and get suitable accessories. What is your GD passionate about, usually at that age they have some favourite - dinosaurs/ jungle book/ pirates/ whatever. Head off to the pound shop - they will have plenty of tat which corresponds with said theme. Don't worry too much - make the party a limited time - 2 hours is enough - plan a few games - have tea- then wind down with a few more games finito!

Choose a current theme . Maybe Pokemon , or Avengers or something more simple like cars or trains. I'd avoid fancy dress as that makes it all a bit complicated . You can theme your cake accordingly . Don't forget party bags ! They usually have bits and bobs in and some sweets , and obviously a slice of cake .

You can get some themed party ware on line cheaply or from one of the major supermarkets. 4 year olds love games and they can be old favourites like pass the parcel or musical chairs . Have you looked at Pinterest for inspiration and easy ideas?m

I think birthday caps are a US invention! This person is obviously advertising...

What are birthday caps ??
Go for fancy dress if you like but you still need to plan the entertainment and food. A cake is a must for a four year old