nimo, we need a little more information before we can help you. first thing to figure out is whether this beer is supposed to be weak. if it's not, we would like to help you figure out where you went wrong or what process could be improved.
to answer your question, you can definitely add more sugar to boost alcohol. however, you should be careful about how much you add - too much sugar can make your beer very thin and "hot" (taste like solvent, paint thinner, etc). so instead of a tasty weak beer, you'll end up with a disgusting strong beer... i know which one i'd rather have. you can add sugar in either primary or secondary, although primary is best.
you could add a can of malt extract, although i've never done this. you're only 4 days in so there is enough yeast around to munch through it. after you add it, you may want to gently swirl your FV to rouse the yeast that have fallen to the bottom. be sure to do this in primary, NOT secondary!
no matter which option you choose:
1) do it as quickly as possible, tonight if you can
2) boil the sugar or malt in some water to sterilize it. do not add it directly to primary. boil for 5 minutes, let cool (putting the pot in an ice bath will speed things up), then add to primary. if you go with sugar, add just enough water to absorb all the sugar. if you go with malt... i don't know how much you should use, but you need to be careful that you don't burn the thick syrup. i'd say at least as much water as extract, maybe more.
good luck, be sure to come back and let us know how it goes.