.NET framework versions are a kind of confusing because 2.0 and 3.0 and 3.5 are all really framework version 2 and .NET 4 and 4.5 are framework version 4.
If your hosting account supports framework 4 then somewhere in your web control panel there should be a place to select .NET 2 or 4. If you don't have any option like that contact your host and see if they offer .NET 4 hosting, ideally you want .NET 4.5 but again you will never see 4.5 listed in the control panel it will still say 4.
The different framework versions require different application pools so that is what the setting in the control panel does it selects an application pool for the correct framework and that makes the site use the chosen framework.
To use mojoPortal hosted in .NET 3.5 (which is the oldest version we support) you would use the mojoportal package for net35, to use mojoPortal hosted in .NET 4 you would use the net40 package of mojoPortal.