Going down a long hill, I wanted to let the engine help with braking, so moved the shifter over to the corner at the 4 position. The shifter will not stay there on its own, it pops back to D as soon as I let it go. Is there some secret I'm missing?

 

Sounds like the selector lever is either bent or disassembled/reassembled incorrectly?

The lever is sometimes bent when the battery dies and the lever is 'yanked' out of PARK.