I agree with Andy, I have beveled many 4X4 post tops with a skilsaw and never looked back, just basic carpentry. You could even make a jig to run the saw across, a 3 sided one with a larger top and a square straight edge to guide the saw, if you want to get fancy. I would cut the bevels before I cut them to 6" lengths too as they would be easier to handle. Maybe too late for that. If you have already cut them to length, I like the previous suggestion of lining them up in a row and beveling one side of all of them at once, that only makes 4 cuts to do them all.Personally I'd happily make that chamfer cut on a 4x4 with a hand-held circular saw, probably just with an aluminium roofing square clamped to it as a guide. I often cross-cut 4" wide boards with a 6.5" circular saw and I've found it works fine without needing any other support or guidance.