In order to make the slope, use leveling feet (as mentioned by @dennyjones). You can make these adjustable from the top by double-nutting them. This would allow adjustment from above using a socket wrench. The range of adjustment you are looking for is only 2.4" (to get a 4% cross slope in 5'), so this should not be difficult to achieve. From above you would have roughly a 1/2" diameter hole in each corner to access the adjusters.
Also, run your studs perpendicular to the length, since the common stud length is 8'. This will also make it easier to get a planar surface. (Separately, in the previous green you built, I would have laid the plywood 90º from how you installed it so you wouldn't have to land two pieces of ply on a single stud's edge.)