Some care needs to be taken with what you plant - you say the slope is four feet, and the wood dividers appear to be about a foot apart each. If you choose the wrong plant for the top, you could end up with it cascading right down to the bottom, over anything else you've planted in the lower parts.
Heucheras come in a variety of leaf colours and do well in dappled shade. These won't necessarily cascade, so they'd be good in the top part or the one below. Campanula portenschlagiana (also known as C. muralis) may well cascade a little, but not enough to drown out anything below it - its evergreen and a neat grower, unlike its cousin, Campanula carpatica. Iberis sempervirens will spill over to the level below, again evergreen in all but the coldest of winters, white flowers in spring. It prefers sun, but will grow in dappled shade. Saponaria ocymoides and Cerastium tomentosum are worth considering - again, they prefers full sun, but I've seen both growing quite well in dappled sunlight. These will cascade and should both be cut back after flowering is over to keep the plants bushy. Silene schafta flowers in late summer, and may also tolerate dappled shade - will cascade, but not too much.
Euonymus 'Harlequin' should do there too - just an evergreen with variegated leaves, hugs the ground quite closely, slow growing, will eventually spill over the edge. Sarcococca hookerania var. humilis makes an evergreen shrub, neat grower, scented white flowers in winter, ultimately 2 x 2 feet, does very well in shady situations. It won't cascade, but will give a little height to the planting.
All plants mentioned hardy down to USDA Zone 5 as a minimum.