Does raising speaker cables off the floor really make a big difference?

Cable raiser manufacturers also sell a special gas mixture in pressurized tubes which you spray in front of the speakers which increases the frequency response to from -50hz to +1 megahertz.  Especially the negative frequencies are of enormous importance to sound quality.
The frequencies above 100k are reserved specifically for the people who can hear the difference the cable raisers make.

About 3-4 inches

In fact, go one step further.  Enclose all your cables in a vacuum pack.  The air around them causes the signal to deteriorate 0.000000000000000000000000000000000000001%.  This is a HUGE degradation by audiophools standards.