Briefly, for a 4 page leaflet, you need to set up two facing pages with the intent of print. The layout would be pages 4 and 1 on one of them and 2 and 3 for the reverse side. Are you going to print this yourselve on a desk-top printer, if so, do you have a duplex printer or are you going to pass it to a commercial printer?
In either case, work in sRGB color mode, ensure your images are Placed in InDesign and have an effective resolution of around 300PPI. Print from a PDF, not from InDesign.
And get a training book such as InDesign Quick Start Guide and/or take some video tutorials such as those on Lynda.com and TrainSimple.com
In New Document setup choose Letter-Half as the page size, with Facing pages checked and the Number of Pages as 4. That will give you a cover (page 1), inside spread (page 2-3) and back (page 4).
Use Print Booklet to print the "printer" spreads on the front and back of a letter sized sheet (4-1, 2-3). Here I have the Print Settings paper size set to 11" X 8.5":