New oven to old junction box
If the current wires are individual wires in conduit, you could pull a new wire or wires to give four wires of the correct size.

But if the wires are in a cable then unless the instructions with the oven explicitly state how you are to connect this oven to the type of electric supply you have described, it seems to me that you cannot connect this new oven to the old wires.

The old cable with B, W and bare copper is for an appliance requiring only 240 V carried by two hots--B and W. The bare copper is to protect against a fault and does not carry current in normal operation.

The presence of the four wires in the new oven suggests that it requires 240 V and 120 V. The 240 V is between the B and the R, as the two hots. The 120 V is between the W (neutral) and one of the two hots. The W neutral is not "hot" but it will carry current in normal operation.

You will have to remove the old cable and replace it with a 4-wire cable of the correct size.

EDIT

If the current wires are individual wires in conduit, then you can pull another insulated wire (you'd use red) of the correct size for this new oven.

And if the current wiring were in a cable but with three insulated wires in one cable (B, W, R), then you could use the existing cable and add a bare copper wire as a ground for a fault condition, but with only two insulated wires you would need to add another insulated wire which would carry current in normal operation. All the wires which carry current in normal operation must be in the same cable or in the same conduit.