Any locals know how thick the mud is in the river the remains were found in?
There was a guy in TX that jumped into a river and got stuck in the mud and drowned a while back:
How a Texas man was killed by quicksand on the San Antonio River last year
If they all four jumped in at the same time legs first and got stuck in the mud up to their waste with their head below the surface of the water, they’d be in a heap of trouble.
If two jumped in and became distressed, the other two may have jumped in to try and save them.
I know it sounds crazy that four grown men could all drown right next together, but it reminds me of the family in California that died of heat exhaustion while on a hike -- their bodies were found within yards of each other. Accidental drowning is the likely CoD given the circumstances and what we know so far, IMO.