I've been lifting weights for about 3-4 weeks.low weight/high reps. i've gained 4 lbs.why is that? it can't be muscle gain. i thought i'd lose a few.

Tailor your exercise: regimen to what you want to accomplish. For weight loss, you need to do cardio in addition to lifting to burn calories. For tone and flexibility, more Yoga and Pilates. For muscle mass building, high weights less reps. However the key with all of these fit activities is NOT TO EAT MORE. It's tempting when you begin exercising to think that you are allowed to eat whatever you want now. Don't!

Muscle weighs > fat: Most likely it is that you have gained muscle! You may have lost fat but can not tell by the scale so you need to get a body fat analysis machine to see how much body fat you lost. But since you probably do not know what your body fat was prior to testing, you will not really be able to tell if you lost fat. But muscle is more compact than fat so your clothes should fit better. Order a consult.