First, unless there is something wrong with the existing cable for the wall oven, I wouldn't bother replacing it. Yes, 4 wire is marginally safer than 3 wire, but not enough to bother with replacing it (in my mind).

If you do decide to replace it, you'll need to describe more about where the white wire was "cut shorter". If this is inside the junction box, then yes, you can create a pigtail to extend the white oven wire.

If by "steel conduit" you really mean metallic sheathed cable, and if you have slack in that cable, then you could certainly shorten that cable to make all of the wires the same length again.

All good electricians have a wire stretcher in the bottom of their toolbox for such situations.

Murphy's Corollary #12: Wires cut to fit, don't.

Thx for the replies. Yes the area I am talking about is in the junction box. I have read a lot about this and yes I agree that cutting the wire shorter was a mistake. I just copied what was done with the old oven. It is done now so this is what I have to work with.

I talked to my electrical inspector for the city today and he said the wiring is fine the way it is. New 4 conductor is better and code for new construction but 3 conductor is fine and the wire being alum is fine as well.

I talked to 2 electricians at work and they said they would replace it so this is where this all started. So now I am thinking of just keeping the current setup with a crimp connector just like it has been for the past 55 years.