Hi Chris,
It is not necessary to remove the 3.x when you go for installing 4.x version.
Please refer the following Kb article that shows how the Syncfusion installation can be switched between various versions.
http://www.syncfusion.com/support/kb/studio/Default.aspx?ToDo=view&questId=36 In Syncfusion Essential Studio 3.x version, the assemblies are automatically referred to 'Assemblies' folder by default. If that folder was renamed or removed, it refers in GAC.
In Syncfusion Essential Studio 4.x version, assmblies refers to public assemblies folder by default. If the assemblies are removed in public assemblies folder, it refers in GAC and precompiled assemblies folder.
Kindly try this and let me know if this helps.
Thank you for using Syncfusion products.
Regards,
Kumaresh