I'm going to take a wild guess and say that's the bottom.

And I say that because 83kHz is a strong signal meant for deep water, and you're in 4' of water, so you're getting ping rebound (double image overlap) from the shallow bottom. Remember ... any image or colors BELOW the onscreen depth mark are NOT what's on the bottom !! The only thing that means anything is the image/colors ABOVE the line of actual depth. In your case, the 4.5' line (or actually just slightly above that line, since your finder reads the actual depth at 3.9ft) ... and everything below that is ping echos.

Either that or you were in 10' of water and some really TALL & THICK grass (and the grass is so thick the unit thinks the top of them is the bottom)

Just one of the reasons I never looked back or missed 2d sonar, once I got a Down Image unit.