files1 = list.files(path2, pattern = "*.tif", full.names = TRUE)
files2 = list.files(path4, pattern = "*_cmask.tif", full.names = TRUE)

for (f1 in files1) {
    f1Stack = stack(f1)
         do stuff with f1Stack
    for (f2 in files2) {
        f2Raster = raster(f2)
        do stuff with f2Raster
    }
 }