There are some templates out on the internet to do this... but .... if you are just doing it once, it's probably easier to just do it yourself rather than figuring out how to download and install template, test it, replace if not doing what you want.
As chrfr already noted, you may have to put the page back into the printer and print on the back to finish the job. It is important that you pick up the printed page by the short edge when flip it . Other wise you are flipping it side-to-side and not end-for-end. When you finish step one (below) experiment by flipping from the long and short edges and you'll see why.
1) Take a blank piece of paper, fold in half. Use a pen and write the numbers 1 to 4 for each page, in the order that you want them to appear, to create a page guide.
2) Create a document in Pages in landscape mode. Use Page Layout mode.
3) You can get fancy with columns and gutters, etc... or you can just create some text boxes, sized to the booklet pages. Insert a 2nd page while you are at, and create two more text boxes.
If the layout is more complex, just snap a guide down the middle of the page and maintain a consistent margin on the page.
4) Use your page guide (step 1) to insert the content in the right order. Depending on the content you can link the text boxes so that the text automatically flows from one box to the next.
5) Print page 1. Flip paper (by the short edge) and print page 2.
Walla! You are done....
Good Luck.
If you need to do this a lot there is a PDF booklet maker that automates a lot of this. But it is useful only if you are making a lot of booklets, or making a booklet with many pages.
