No, the EmBlaser's grbl has a feedrate curve. What that means, it's not linear with a set feedrate range. If you set it at 4000mm/min, it may actually be only running at 3500mm/min. With a 4W you really need to move fast on Leather, but there is a limit how fast the EmBlaser's grbl can run.
In the image editor, there are 11 different Dithering Algorithms to choose from. Open your image and try Atkinson, then save your image again.
Reopen in PicLaser and set your max at 40? and min at 0. A Dithered image is only 1Bit B&W, so it will pulse the laser ON/OFF with only two laser power values.