If you are pursuing the MCTS:Web developer, I suggest that you go straight for version 4.0. This requires a single exam (70-515) rather than the two exams required for version 3.5. It is true that you will need to learn some new things, such as MVC and jQuery,
but they are quite useful and in great demand, so the time you spend on them will be well spent.
Note, however, that in both cases (3.5 or 4.0) the exams will be retired on July 2013. As an alternative, you can consider certifying on version 4.5 (visual Studio 2012). The exams for this version can be found here:
http://www.microsoft.com/learning/en/us/mcsd-web-apps-certification.aspx