You need to change the entire cord on the oven. You manual from the oven or the manufacturer web site should have instruction how to do it.
What you need to do is look where the current cord attaches. The red and black will be the same. The white and green need to be split apart to separate connections. There is probably a jumper in the stove connecting them together now.
On the back of the oven, take the cord cover off and you should see a 3 terminal block. The outer 2 should be the 'hot' terms, and the middle the neutral. See if there is a jumper or bonding strap between the center term and the frame/casing of the oven. You'll want to remove that strap/jumper. If there is a screw holding that strap to the frame, use it to hook up your 4th wire, which is a ground wire.