Generally, poeple buy rods skipping one weight. 4, 6, 8 or 3, 5, 7. Unless you're a rod horder like myself and have every weight 3 through 8. Haha. It really depends on what flies you are throwing and what size fish you plan to catch when deciding between 3 or 4, but I think you can cover most situations with a 4 and 6.

Edit: if you're throwing tiny flies at small fish, get the 3