I had posted the info below in an earlier thread;
I currently have the Studio 60 v.2 and have had a chance to listen to the v.3 and v.4 incarnations as well.
From the technical stand point, the v.3 has a and improved cabinet structure, 7' midrange drivers instead of 6.5' or 8', the midrange also includes a phase pug for better dispersion. The bass drivers also move the a consistent 7' driver instead of 6.5' or 8' drivers. The tweeter is also a new design and has been anodized with satin to improve rigidity without a weight penalty. All drivers are mounted using a gasket to separate the drivers fromt he cabinet called IMS shockmount. New crossover design, improved binding posts, and a die cast port.
The v.4 update is a change in diaphragm materials of the tweeter to the gold anodized aluminum used in the Signature speakers. This does not make it the Signature tweeter as there is a few more improvements in a variety of areas for that driver. The midrange diaphragm also changes from the mica loaded polymer to aluminum with satin anodizing.
Sound wise, its very evolutionary. The v.3 change being the major update in design exhibits improved imaging, transparency, and tonal balance. The v.4 update takes a small but audible improvement in transparency with slightly better controlled highs.