In Word, open a new, blank document.
Go to File > Page Set Up > Margins
Set 'Multiple Pages' to 'Book fold'
Adjust margins as required.
Set 'Sheets per booklet' to 4
Click 'OK'
You'll now see the front page of your booklet. As with a normal Word document, page 2 won't appear until you type enough text to go beyond the end of the first page or (probably better) you insert a page break.
Once you've designed all 4 pages, printing the document will automatically place page 4 on the left hand side of an A4 sheet, with page 1 on the right. Similarly, pages 2 and 3 will appear side by side. Depending upon your printer, it might take a bit of experimentation to get pages 1 & 4 printed on the reverse of pages 2 & 3. (It will depend whether your printer supports duplex printing. Even if it does, you might find that you'll need to select 'manual duplex' and flip the paper over).
Far simpler, use a decent DTP program. Serif PagePlus SE is completely free and will make life a lot easier for you:
http://www.freeserifsoftware.com/
Chris