Users PC installing wrong version of Silverlight

I have a Silverlight Application (SL4) which I have rolled out to people with开发者_JS百科in my network. The problem I have is that when some of my users go to access the SL application it is asking them to download a Silverlight 3 plug in.

Is there something I need to set in my Project to ensure that the Plug in installs SL4.

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Sounds like the host page HTML is incorrectly set. Your object tag HTML ought to look like this:-

<div id="silverlightControlHost">
    <object data="data:application/x-silverlight-2," type="application/x-silverlight-2" width="100%" height="100%">
      <param name="source" value="ClientBin/SilverlightApplication1.xap"/>
      <param name="onError" value="onSilverlightError" />
      <param name="background" value="white" />
      <param name="minRuntimeVersion" value="4.0.60310.0" />
      <param name="autoUpgrade" value="true" />
      <a href="http://go.microsoft.com/fwlink/?LinkID=149156&v=4.0.60310.0" style="text-decoration:none">
          <img src="http://go.microsoft.com/fwlink/?LinkId=161376" alt="Get Microsoft Silverlight" style="border-style:none"/>
      </a>
    </object><iframe id="_sl_historyFrame" style="visibility:hidden;height:0px;width:0px;border:0px"></iframe>
</div>

Note in particular the minRuntimeVersion param and the "&v=4.0.60310.0" query string value of the href in the object fallback content.