So what is wrong w/ the 4x4? I'd use a PT 4x4 and galvanized (unless it reacts with the plants) or stainless wire; I'd expect that to be cheaper than aircraft cable. Either drill holes in the 4x4s to run the wire through or run the wire to screw eyes or hooks (galv or stainless). You'd be able to easily tighten the wire w/ eyes or hooks.