Another option!
Select File >Page Setup >Portrait.
Then without coming out of Page Setup, select the Paper tab and select A5 paper size. You may wish to adjust the margins as well, otherwise you get a lot of space and very little writing.
Click ok.
Select Insert >Page numbers. Put them where you want them.
When you have finished typing, select File >Print >Manual Duplex and also select 2 pages per sheet at the bottom right of the window.
It will now print 2 pages of A5 on one sheet of A4, starting with a sheet which is on the right hand side and subject to your printer's capabilites, allow you to print one side then turn it over and print the other etc. You may have to mess about with the order that you print the pages in order to collate them, but you can specify which pages to print, e.g. 1 and 8, 2 and 7, 3 and 6, 4 and 5.