Once you have successfully upgrade to version 4.0 by referencing to new assemblies, you may want to check the WebCombo's appearance in your existing web pages. If you are experiencing inappropriate display of the WebCombo's appearance, most chance you were using one of the styles listed in Styles breaking changes. Please read the Breaking Changes topic to learn what have been changed since previous version. The resolution to the styles problem are also discussed in the sub topic.
See Also
Other Resources
Upgrading Overview
Assembly Reference
Checking Layout after Upgrading