6-8 wet diapers in 24 hours is the general guideline. I'm a peer breastfeeding counselor with Nursing Mothers Counsel, and I'm curious to know why you feel like your milk supply isn't enough for her. I don't know how dedicated you are to breastfeeding (and that makes a big difference), but at 4wks, she should be nursing 8-12 times per 24 hours. There are so many unknowns, though, regarding how much, how often, latch, the start you got, etc.
Weight is also a good guideline for how she's doing. Most babies gain 3/4 - 1oz per day for the first 3 months. However, as long as her weight gain is consistent, it doesn't have to be right on that number.
Also a good guideline is how she acts when she's awake and if she wakes herself to eat. Does she let you know when she's hungry? Does she have clear hunger cues? (arm/leg cycling, smacking tongue/lips, fist-to-mouth, bobbing, rooting) Is she alert? Does she make eye contact when awake? Is she generally happy? Does she seem comfortable? Does she smile, and is she hitting her milestones so far? All these are good indicators as well.