hmm year 4- ok i have been a preforming arts/ music teacher in previous life and have put on heaps of school concerts - so ask away at any thing you need. First question - how much time does the act have to be ??

Here are some ideas
A circus theme- we got some circus people in to teach them to spin plates, juggle, devil sticks but it wasnt hard and you could teach them themselves. The gym kids did a gymastic/dance to the circus theme in there and each child got to star for part of the act - even for like 10 seconds lol. Choose a piece of music and mix it together and put the whole act to that

An olympic theme- topically due to the olympics
we did a mix of sport songs and the kids dressed up to the sports and acted them out in a dance- such as kung fu fighting, rocky, surfing safari, etc- mixed a section of each song together so you can make it as long or short as you like

We also have done music acts- recorders/marimbas etc. Even made out own instrumemtns and own compositions etc and performed them

Let me know if there is a theme you need to tie in with, the length you want the act and how many kids and i can help you plan it!!

Keep it moving, no repitition, little or no stops in between sections and keep it light and fun
let kids be kids!