Slope is usually expressed as the rise, or vertical change, over a specified horizontal run, slope = rise/run
If expressed in %, then slope (%) = 100*rise/run. Some people further confound it by using inches of rise per foot of run, then slope(%) = 100 *(rise/12)/run, where rise is in inches and run in feet. In this case, rise(in) = (slope%/100) * 12 in/ft * run(ft)
For your example 0.4%, in a 1 ft run, the rise is
0.4/100 * 12 * 1 = 0.048", or 0.48" in 10'