Comparing the two on Vizio's Web site, it seems the main difference is that the XVT3D4SV is 3D-compatible, meaning it can play 3D video programming when watched through optional active shutter glasses. Vizio sells the glasses, model VSG-102, in a two-pack for about $200.
The XVT3D4SV is Vizio's only series to use active 3D technology rather than the passive method used by the XVT3D650SV and forthcoming 2011 Vizio models like the XVT3D5 series. See CNET's 3D TV FAQ for more information.
The only other differences in specs aside from 3D are refresh rate (480Hz on the 4SV versus 240Hz on the 3SV) and response time (4ms and 5ms). We don't expect these differences to have much, if any, visible impact on picture quality (more info).
Otherwise we expect the XVT3D4SV to perform similarly to the XVT3SV with 2D material. For more information, check out our full review of the Vizio XVT3SV series.