I've found that the cold weather over recent months has brought my Mk4 down a tad. When I got it, it was recording a very consistent 760 ft/second with 8.4's. Comparisons with a friend's 785 ft/second RN10 (same pellets) showed no difference in windage at 50 yards in a stiff breeze.
The small adjuster on the left will give sufficient adjustment to get you above your current level...a half turn gave me the required 20ft/second, but don't get too hung up on power; it just isn't necessary. I managed to qualify for AA grade in my region with mine doing 10.6 ft/lbs.
Oh, and when the warmer weather eventually arrives, it'll go up again, and may require downwards adjustment.
On balance, I wouldn't touch it if it's consistent.