In brief - I want to use a 2x4 to resist the tension put on trellis posts ~16ft apart, rather than the other two options.
I'm building a grape trellis with upright 4x4s connected by high tensile wire. As with any wire setup, like a long wire fence, you usually need to brace the ends to avoid leaning. I'm thinking I can just run a 16ft 2x4 between the posts at the top, using brace pins to attach to the posts. This 2x4 wont really hold any load, as that will be held by the high tensile wire. The 2x4 is only there to resist the tension force that the wire puts on the main 4x4 posts, turning it into a compressive force on the 2x4.
Can a 2x4x16 even make that span given it's own load? And if so, is that compressive force (it should be straight and level) going to force it to sag?
FWIW this is really to avoid the other two options - 1) angle the posts at 60deg so the tension forces the posts into the ground, or 2) build large H braces on the ends.