I've floated the river when the POR guage read above 9'.
Caught fish, had no issues until I felt something scratching the bottom of my boat and noted it was a submerged TREE
Then I realize that may have not been a good idea.
POR @ 5' is pushy, fun, predicatable for fishing, but pushy.
But POR @ 4' can be more dangerous depending on your skills. You float over nervous water and a ledge or rock is just barely covered, not paying attention, then a problem could arise.
Low water levels running rapids offer the opportunity to flip more than high ones because when the water's high, you have a pillow of water over the edge a yak can float over.
Only you can decide what's safe for your comfort in a yak. And the only way to figure that out is to know what the river level is where you'll be floating, then do it. Keep a log and you'll figure out quickly what's safe for you.
But I'll still stay POR @ 5' is pushy.
One of the reasons why I have a pontoon boat. If the water's up, and I'll be in an area with allot of rapids, I'm in the toon.