It really only needs to be deep enough to hold the appropriate amount of soil for growing what you want to grow, I think. Ours is only about 10" and does fine. If I go 4' wide, I like to be able to access it from both sides so I don't ever have to walk in it...keeps the soil from compacting. If I had a long, narrow space like that, I'd think about putting 2 feet on either side with the grass or whatver is on the ground in between. That way you can kneel or sit on the edge and reach to the back for maintenance.