The number of pages in a brochure is always a multiple of 4, with a minimum of 8 pages. Why is that?
Brochures are printed on large sheets, which are twice as large as the closed size of the brochure. These sheets are then folded and attached together (for example with staples). Each of these sheets contain 4 pages, 2 on the front and 2 on the back. This means that there's always a multiple of 4 pages.
The minimum of 8 pages has a similar reason. If a brochure would only consist of 4 pages, we would only need 1 sheet (2 pages at the front, 2 pages at the back). If there is only 1 sheet you cannot attach it to anything (e.g. stapling). If you still want 4 pages, you should order a leaflet with a single fold.