So a low obstacle usually grants cover if you are within 30ft and closer than the enemy in question. What happens if you are Prone behind a low obstacle?
Logically you'd not be within line of sight, so you'd have total cover. But I'm not sure that's how it works in the game.
A low obstacle is a wall half your height, so I suppose being prone means that you are in total cover (since as you say the enemy has, logically speaking, no line of sight on you). This could result in a cheesy tactic though as you can rise from prone (move) -> shoot (standard) -> drop to prone (swift) in a single round. Then again, you pay for that tactic in sacrificing movement, not being able to full attack and being more vulnerable to melee attacks should the enemy reach you. You can liken this to someone hiding behind a wall and quickly popping out to take a quick shot. This could of course be countered by the enemy simply readying an action.
I'd have to say you're right that prone behind low obstacle is total cover, if anything based on common sense. I don't see why a person crouched behind a barrier that covers their entire being would only get a +4 AC (plus the prone bonuses).