For 3-4' walls I would recommend allen blocks. Get the large style, about 18' wide by about 8" tall. They simply stack upon one another and lock in. Simple as legos and more importantly they do not retain water. Put some filter cloth and a bit of gravel behind the wall, and when it rains, the water slowly leeches right thru the wall. The biggest cause of failure in retaining walls is people try to make them watertight and heavy duty. Water pressure will destroy anything. You must either build a proper drain tile system or use a wall like the allen block that let the water pass right thru. I did a 4' wall by 80' long. About $900 in material and your labor.