I've recently been trying to switch my project over to 4.0 and came upon the following problem:

Error   10   The .xap file was created with a version of XACT that is incompatible with the XNA Framework Content Pipeline version used by this project. Refer to the documentation for options to resolve this mismatch.

The Xact Project was built in XACT3 and was working fine in Game Studio 3.1

Anyone know how to solve this problem?

Thanks,
-Thut

XACT must have changed from 3.1 to 4.0. You probably need to upgrade the .xap file so that the 4.0.0.0 content pipeline assemblies can handle it.

Correct, the version of XACT changed between XNA Game Studio 3.1 and 4.0.  You'll need to open your .xap project in the version of xact3.exe that ships with XNA Game Studio 4.0, click yes when prompted to upgrade it, then save it and close it.  After doing that, you should be able to build it in XNA Game Studio 4.0.  The steps will be similar to the ones in this blog post.  I'll work on a similar blog post with the exact steps for XNA Game Studio 4.0 in the near future.

Thanks!

I wrote an updated blog post at http://blogs.msdn.com/b/astebner/archive/2010/09/23/10067171.aspx with specific instructions for upgrading XACT projects used in XNA Game Studio 3.1 to the format required by XNA Game Studio 4.0.  Hopefully this helps.

Thanks!

Problem Solved! Thanks guys.

Thanks for the information, blog post, and explanation.  Much appreciated.

You're correct - the major and minor version is 3.0 for the versions of the XACT tools used in XNA Game Studio 3.1 and 4.0.  The XACT team introduced an incompatibilty in their project files starting in the February 2010 DirectX SDK, but they decided not to reversion the XACT tools to a new major or minor version.  XNA Game Studio picks up XACT tools from the DirectX SDK and doesn't have direct control over what version numbers are used by the XACT tools.  I'm sorry for the confusion that this scenario has caused.

Thanks!