Jonathan Clark aka Mr A Carbon would be best to answer this, but my best guess would be that the price would be similar.
As no "official" price has been released for the BJ4 to S4 conversion it's hard to say, but, I would guess the conversion is at least £200. This would make a combined price of Bj4 £350 + conversion £200 = £550 which is the price I have heard quoted for the S4 complete kit. The S4 kit also appears to have a better spec than the Bj4 as the S4 comes with a one way front diff and a few other hop ups as standard.
The S4 comes with spare front wishbones and a spare set of gears aswell
