Don't have them change it to 2.0 that will not be work if you uploaded the .NET 4 version of mojoPortal. It might solve it if you are really using the 3.5 version of mojoPortal.
I'm not sure from what you said if you already installed the .NET 4 version of mojoPortal. I think part of the problem is that Arvixe installs mojo from Web App Gallery but that version is for 3.5 .NET and is not ideal for .NET 4 hosting. After Web Platform Install 3.0 comes out of beta our package in the Gallery will be changed for .NET 4 and things should be smoother.
I would keep it at .NET 4 with integrated pipeline mode and install the .NET 4 version of mojoPortal if you haven't already.
Upgrading is fairly straightforward instructions, download the mojo package for .NET 4 from Codeplex then upgrade as described here:
http://www.mojoportal.com/upgrading.aspx
Then at least you will have the .NET 4 package of mojoPortal under .NET 4 hosting and if there are any problems it should be easier to resolve.
Hope it helps,
Joe