I own both a Vertex3 256GB and a Vertex 4 256, and people usually ask me if the vertex 4 is really faster than vertex 3, or difference in performance are really little. I think it is difficult to answer this question, I can only post you Cristal Diskmark test for both drive when they are completely empty.
This is the Vertex 3
And this is the Vertex 4
From the test I can tell you that the Vertex 4 has a great increase of performance in Write Speed , and a real gain of performance in 4K QD32 tests. I had also a Vertex 2 and I was really satisfied with performance and reliability of OCZ drive, and I should admit that the Vertex 4 performances are really really huge.
Do not forget that such a drive needs a good Sata controller or you can suffer performance loss, as I described in an old post.
If you work heavily with Visual Studio or you run quite often Virtual Machines, having a fast SSD is a must.