I passed 70-340 - Implementing Security for Applications with Microsoft Visual C# .NET on Monday. Its a very interesting paper, would strongly recommend anybody who has cleared the 70-315/70-316 and 70-320 exams to take it up. If you've studied for security for the other three papers then you should pass this one with minimal studying.
The reading material I used was
- Course 2300: Developing Security-Enhanced Web Applications
- Course 2350: Developing and Deploying Secure Microsoft .NET Framework Applications
- Course 2840: Developing Secure Applications
- Visual Basic .NET Code Security Handbook by Eric Lippert
- Ofcourse the famous SecNet.pdf on MSDN