Team Foundation Server 12 API with .NET 4.0

Question

So we've run into another issue, and are hoping the combined knowledge of SO can congregate and come up with some sort of a solution.

We're running a program that has a 'Tasks' system. A person in the company, using the software, might have a request, or a bug fix they wish to create a new task of. So they do it through our current task system. Now that we've implemented source control into our dev team, we have the fantastic functionality that TFS provides - Branching, Merging, Conflict resolution, and of course - the Agile side of things - sprints, scrums, etc.

Now we are trying to move our tasks system to a new level - people reporting these bugs/add ons will create a task, but this will in turn move that into our workflow on the TFS.

So we've nearly got it working. The program however is based on the .NET 4.0 Client Profile Framework, and we hit this issue:

C:\Program Files (x86)\MSBuild\12.0\bin\Microsoft.Common.CurrentVersion.targets(1635,5): warning MSB3274: The     primary reference "Microsoft.TeamFoundation.Client, Version=12.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL" could not be resolved because it was built against the ".NETFramework,Version=v4.5" framework. This is a higher version than the currently targeted framework ".NETFramework,Version=v4.0,Profile=Client".
C:\Program Files (x86)\MSBuild\12.0\bin\Microsoft.Common.CurrentVersion.targets(1635,5): warning MSB3275: The primary reference "Microsoft.TeamFoundation.WorkItemTracking.Client, Version=12.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL" could not be resolved because it has an indirect dependency on the assembly "Microsoft.TeamFoundation.Client, Version=12.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" which was built against the ".NETFramework,Version=v4.5" framework. This is a higher version than the currently targeted framework ".NETFramework,Version=v4.0,Profile=Client".

Now I have a somewhat limited knowledge of how to manipulate something that was built in .NET4.5 to work with .NET4.0, but as far as my knowledge extends - I can set the build framework to be .NET4.5 and it runs smoothly.

However, my question is - is there a way to get this working with .NET4.0? If so, how would I go about it?