Tethers, you are in the right place. We can help.
Right, first off, do you want to make the cake or shall we investigate bought versions too? I'm fairly sure you can buy a butterfly shaped tin if you want to make one or that you could make a decent butterfly shape from a couple of round cakes and a swiss roll. I'm also fairly sure that somewhere will sell one if you would rather do that.
4 year olds are fairly easy to feed as they tend to have pretty basic tastes. Sandwiches are always good (ham, cheese, jam/choc spread) with some crisps, some veg for show, some babybel and those mini cocktail sausages are always oddly popular. Cold pizza slices are always good too. Boxes tend to contain the mess a little and make it easier to get everyone sat down and eating but you do get more waste as they won't always eat everything in their box.
I'll have a look for some invites for you. We might be able to find some pre done printable ones.
Themes are not necessary, they just sometimes make things easier because they narrow down options when there is loads to chose from.
Decorations - For a hired room, I'd go with simple decs. I'm having bunting and balloons for DD's party in December. Balloons can even be a game in themselves once off the walls as kids love chasing them round the room.
Stealth is a the party game queen. The old ones are the best - pass the parcel, musical chairs/statues, pin the something on the something. Simple games that are easy to explain and that they've probably played at other parties are the way to go so that you don't have to spend ages explaining the rules.
Crowns is a good idea for craft. Relatively cheap to buy and can be decorated easily with a few pens and some stick on gems. You want to avoid glue and glitter as you'll be cleaning up for hours afterwards otherwise. You can get things like that cheap from Yellow Moon etc. I did little cardboard butterflies at a party a while ago that went down well with little girls...? Colouring always a cheap and effective activity too. You can print colouring sheets from the internet for no more than the cost of the ink.
A party dress is always nice :)
People will come, just try to give them a couple of weeks notice and make sure you include a mobile number for RSVPs. We got one with only a home number on a few weeks ago and it took me ages to reply whereas I'd have fired off a text straight away.