4 sets 15 reps?
 

I've been training for a year by now, but since I had little progress, I decided to pay for a personal trainer. I usually train two muscle groups a day (example chest and biceps), and what confuses me is that I'm asked to do 4 sets of 15 reps per exercise (I usually do 3-4 exercises for muscle group). For each set I increase the weight (or for the first 3 sets I increase the weight and for the 4th one I lower it). Is that good for gaining mass (thats my goal)?

The 15 rep range is leaning a bit more towards muscular endurance than hypertrophy. I would keep the rep ranges around 6-12 if you want to maximize muscle gain.

Well I thing four set fifteen reps is enough and doing extra exercise is not important rather you should focus on perfect way of exercise in a scheduled routine.

A said, 15 sounds a little high for building muscle, it may work for some but not anyone I've met yet :).

Also, once you find a rep range that works for you, don't stick rigidly with it for all exercises. Change things around to find what works best for you.