there are two reasons that i can see using 4 over 3:
1) if you use 4 cards at 90deg; you can combine any two cards for new patterns, invert the opposite card and you get a figure 8, etc. More of an ambisonic's technique then a DMS.
2) If you do the normal DMS setup, but use an extra hyper card or some other pattern, then you can generate surround sound and use that 4th cap as your mid. So midside FL & FR, midside RL & RR, and then the dedicated hypercard for the FC.
Outside of that, I'm not sure what you'd use it for.