My Experience is with full sized pick up trucks with 500 to 1000 pounds of gear in the bed. I can say that four feet is not enough to float a 1/2 Ton or 3/4 Ton pick up with that kind of load, but I think it also depends on whether there is a current in the water or if the vehicle is going slowly or not.
I have seen green horns hit a ford with 4 feet of water over a distance of 20 feet at mid point, going 30 miles an hour and have watched the vehicle bog down in the middle of the stream. I have seen the same type of vehicle (PU) crawl through the same ford and out the other side (4WD of course). Much more than that will cause the ass end to float first and even then the front wheels still grab. If current gets under the floating ass end and pivots the truck, its game over too. Driving skill matters a lot.
Of couse , as dumb college students with company vehicles we often pushed the limits when we should not have.Our job was to get into inaccessable areas and set up base camps for survey work. Looking back on it, it was a LOT of fun. We all, fortunately, carried hand powered come alongs and heavy rope which would pull the truck out for drying out, if we got bogged down.
The venerable " Dodge Power Wagon" was the best PU for this all back in the late sixties and early 70's.
I once got a Land Rover stuck so bad, it took a D7 dozer to haul me out, so I am no expert either.
1969 Dodge Power Wagon,318, 4 speed standard.
