We return to the northern bank of the small river and follow it toward the mountain, as I described above. In the mountains we climb higher, but we don't go too far into the distance, we don't approach suspicious objects, and we go around them in an arc. Along the way, we don't enter anywhere.
Approximately — we comb through the strip of mountains closest to us from south to north until we find all four specimens, then we can head back to turn in the quest.