VT never used G4s, but they did make a cluster of 1100 G5s back in 2003, which they later upgraded to custom built 2.3 Ghz Xserve G5 Cluster Nodes. (this was before the 2.3 Ghz variant was released to the public, so they were built especially for VT) Its still running too, they lease time on it to outside companies.
linky