If you mean fix compilation errors: No.
If you mean fix logic errors: No.
If you mean fix issue relating upgrading from 3.5 to 4: No.
Here's a guide for [upgrading your 3.5 code for 4][1].
If those don't cover what you expect to be fixed do you have an example of what you think it could possibly fix?
[1]: http://docs.unity3d.com/Documentation/Manual/UpgradeGuide3540.html
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