Honestly, I've never seen a post frame structure with a truss spacing more than 4'. I would imagine it's possible for something small or for when there are unique and heavier trusses, such as for simulating post and beam, but 4' pretty much seems to be the standard for these structures. I've been in dozens of them in the past ten years. Post spacing is also generally setup in increments of 4' for the side walls for sure and when possible, on the gable ends, too, but there's more variability on the gable ends. A post frame builder will make something any size you want, but they "greatly prefer" 4' increments because of lumber standards and how things are commonly engineered.

Interestingly, in metal post frame buildings (think "car port") type for this example, the standard spacing appears to be 5' intervals for both posts and trusses. You can order with 4' (which I would have done had I chosen that building type) which is easier to do interior work because of standard material sizes, but it comes with additional cost. "Red iron" type structures have much longer spans...