Error on potential DP

No error. You can't assume the double play would have been completed. Score runner's out as 4 - 6 and batter with a fielder's choice.

Was the "bad throw" bad enough that it allowed the runner to advance to 2nd base? If so, then you've got a FC and an error -- if not, no error.

OBR: 10.12
(d) The official scorer shall not charge an error against:
(3) any fielder who makes a wild throw in attempting to complete a double play or triple play, unless such wild throw enables any runner to advance beyond the base such runner would have reached had the throw not been wild;