.net version troubles -This assembly is built by a runtime newer
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I enhanced an old website just a bit, but the problem is I used telerik controls which requires a higher version of .net than the original website has. The website is hosted externally, and I only have access to the website itself through an ftp site. 

My question is what is the easiest way to set up this site to be able to handle .net 4.0 but also be backwards compatible? I'm almost sure that the version of IIS on the old server is running .net 4.0.

Thank you in advance..