I'm just guessing here, but I'd say longevity. The 4200 has been available for sale for a number of years, and now that the 4300 is out, seemingly as its successor, I think it won't be too long before we see an EoL notice for the 4200.
The new series has pretty much the same use case. But, as you said, it supports higher speeds. It also has more memory, faster CPU, likely newer and better ASICs to support new features in the future, smaller high-end power supplies. It is also a bit less noisy and has front-to-back or back-to-front air flow (instead of side-to-back in the 4200) which will allow the switch to fit better in hot/cold separated environments.