Your certification is still valid. Contrary to the newest certifications, which require periodic recertification, the old MCTS does not expire.
There is no upgrade path from MCTS 2 to MCTS 4. It wouldn't make any sense, since the
MCTS .Net Framework 4 in ASP.Net Applications requires passing only a single exam.
You only need to take exam 70-515 to achieve your MCTS: .NET Framework 4, Web Applications.
http://www.microsoft.com/learning/en/us/exam.aspx?ID=70-515&locale=en-us