HOLY COW. I've heard of the bars bending, but only when someone held onto them and used them to pull themselves further up into the hammock (Brandon at Warbonnet said he'd done so as a test).
Definitely should not happen under normal circumstances. How far apart were the posts? My only guess is that you had to set the hang angle to be too flat to keep it off the ground since the posts were only 4 feet high. Doing so would probably add more pressure to the suspension triangle and the spreader bars as a result.
You might need to hang it a bit higher than four feet and allow for more sag in the system. It doesn't need the traditional 30 degree hang angle but still should be at 20 or 25. For more on hang angles check out step 1 at this:
I'd definitely contact Brandon at Warbonnet with some details. He's a good guy and there's a chance he'll replace the poles (and would certainly do so if the break was an issue with a quality problem in the pole).