How do I connect a 4-wire oven to a 3-wire outlet?
You'd have to look at the old receptacle to be sure.  If it's a NEMA 10-50, then the following is true: it is being fed by common 6/2 grounded cable. 

NEMA 10 is an obsolete and somewhat dangerous receptacle family, used in the old days for ranges and dryers.  It provides hot, hot and neutral - 240V hot-to-hot, and 120V hot-to-neutral.  It does not provide ground.

Often, this was installed using the common "/2" cable. There's no choice to color; it's always black, white and bare.  In this usage, the white is not a neutral; it's the other hot.  The bare wire is the neutral. Really. 

Today, a white wire used as a "hot" must be marked with tape.  Back in the old days, that was not required if the usage was obvious.

I would make the argument that the old circuit is "grandfathered", which it is.  If the 10-50 receptacle broke, you could change it without breaking the grandfathering; in fact the stores sell 10-50R's for only that purpose. I would argue the same is true for changing it to a modern 4-prong NEMA 14-50.  I would then use the NEC 2014 rules which allow retrofitting a true ground. Can't promise you the inspector would agree.