I am wiring a new stove in an old house! Using 4 prong stove cord and will install 4 prong receptacle and flush box at wall. Problem is how to wire new wire (6/3 w ground) into old sub panel breaker box. Original was hard wired using using 3 conductor BX and I am not sure as to where to connect the new 4 wires at the breker box. It is a 50 amp breaker, so should I pair the green and white at sub?

Original was hard wired using using 3 conductor BX and I am not sure as to where to connect the new 4 wires at the breker box. It is a 50 amp breaker, so should I pair the green and white at sub?
NO!
In the panel should be a separate ground and neutral bar. Bare goes to the ground bar and white goes to the neutral bar. Black & red go to the breaker.
At the receptacle, bare to green, white to silver, black & red to gold.