if you can get 4 complete reps on your first set, your not doing too bad. But you fail to mention your goals, are you seeking size or strength?
If your looking for size, do your pullups to failure while supersetting with pulldowns and get ~8 reps. 3-4 sets of that should be sufficient. If your going for strength, give more time between your sets and try to get a total of at least 30 reps using however many sets it takes. Well, 30 might be a bit high at first, but work up to it.
Also on a side note, I personally hate db rows, i feel barbell rows are superior in many ways... range of motion, usable weight and correct posture.