
Originally Posted by
znak
Doing high rep and low rep both build muscle. Low rep adds volume, high rep adds density. You need to do both.
Since you are asking this question, you probably have not been lifting for long or have at least not been thinking about your lifting for long. I would stay in the 10 rep range for four set for a while until you have a good foundation. Ten reps mean that on the first set you could do more but you don't and on the last two sets you can't do more than ten. It takes a while in your training to understand your body's ability to do each exercise correctly.
As you become more advanced, you will need to do stuff to trick your body into growth. One way to do so is to adjust your reps. Squats for example give really good results in the 4-6 range and the 15-20 range if you are using the right weight for your ability and proper form.
In general, regardless of the number of reps you are doing, the last one in the set should ideally be the last one you can do.
If you are in the ten rep or lower range and do each set to the max, you are on your way to growth. If you could do another 2-3 reps on every set, your weight is too low for you.