If you are buying 4 grams, crumble it in your fingers into as fine a consistency as possible. Make your little pile of weed uniform in height and width then split it in half, then split each half, in half. You should now have 4 little piles of weed that are roughly the same size. Make 4 firecrackers. It really won't matter if one has a little more or a little less pot in it then the others.
ps
If you continue to use marijuana you will use this splitting process again at some point in the future.