Right. Here I am, typing with fingerless gloves cos it's flipping freezing in my shed office, but at least on a keyboard.
Tethers - where are you? I have a butterfly shaped silicone cake mould (it's a bit strange, TBH - it's this one and it's like a jigsaw (but I guess that does make it easier in that it should rise evenly, etc)
Games - 4yos - don't be too ambitious. They don't really know many games yet. However, if they are all at school/preschool together then it is well worth interrogating DD as to what they games play at school - DC who don't know how to play musical statues may know duck duck goose as my DC do although they have never managed to explain it to me.
The reason I opt for themes is that they let you take deeply ordinary games/activities/food and make them special and memorable and they help me focus, but they are by no means compulsory.
I think Blathers has covered everything. 4yos eat a lot less than you think at parties (and a lot less than they put on their plates
), but there is no such thing as too many cocktail sausages, especially if there are adults, and large quantities of mars bar krispie cake / rocky road / other similarly calorific stuff tends to get consumed by the adults at out parties.
Invitations - I like the ladybird book idea - one of DD's friends used some of these for invitations and more for Thank you letters - would that work? You could mount them on card and print/write invitation text on the back, or just write on the postcards. For school/preschool I would give ~2 weeks notice, and make sure you include lots of contact details (home phone, mobile, email) for RSVP and set a deadline ("RSVP by 10/11/12")