If you are using 'assemblyBinding' in your Sitefinity web.config, you will need to update that section of the config to the latest dll versions. Assembly Binding is a common solution to map out of date third party Sitefinity modules/plugins to the current Sitefinity dlls.
The following are correct versions for Sitefinity 5.1.3210. Note that Telerik.Web.UI is a different version as it is from a different Telerik product line.
<!-- From web.config - not complete file -->
<runtime>
<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
<dependentAssembly>
<assemblyIdentity name="Telerik.Sitefinity" publicKeyToken="b28c218413bdf563" culture="neutral"/>
<bindingRedirect oldVersion="0.0.0.0-65535.65535.65535.65535" newVersion="5.1.3210.0"/>
</dependentAssembly>
<dependentAssembly>
<assemblyIdentity name="Telerik.Sitefinity.Model" publicKeyToken="b28c218413bdf563" culture="neutral"/>
<bindingRedirect oldVersion="0.0.0.0-65535.65535.65535.65535" newVersion="5.1.3210.0"/>
</dependentAssembly>
<dependentAssembly>
<assemblyIdentity name="Telerik.Web.UI" publicKeyToken="121fae78165ba3d4" culture="neutral"/>
<bindingRedirect oldVersion="0.0.0.0-65535.65535.65535.65535" newVersion="2012.2.607.40"/>
</dependentAssembly>
</assemblyBinding>
If you will be upgrading through many versions 4.1 -> 4.4 -> 5.0 -> 5.1, you may wish to comment out this section of the config until you are complete with your upgrades, then add it back in. Otherwise, you'll have to edit these three lines after each upgrade.
Note: to get the dll version, browse to the bin folder. Right click dll file, click properties, then select the 'details' tab.