Technically your current installation can run under .NET 4 but as soon as they update your site to .NET 4 it will cause an error because of a change in .NET 4 where the section declaration for system.web.extensions is no longer needed in Web.config because it already exists in machine.config. So what will happen first is that you will get an error about duplicate section registration.
This problem is discussed in the 9th and 10 items on our FAQ page. You would solve the error by removing that section declaration from your web.config file after they change you to .NET 4.
Then there will also be most likely some problems with rendering, that could be solved by adding controlRenderingCompatibilityVersion="3.5" on the <pages element in Web.config
So, you could potentially keep the version of mojoPortal you have if you don't want to upgrade.
That is a very old version of mojoPortal, so it is likely that if you do decide to upgrade there will be some problems with your skin that need fixing after upgrading. We have some notes about skin changes that you will need but it doesn't go all the way back to your current version. But I'm sure we can help you figure out any other problems that remain after making the changes we do have documented.
To upgrade a site this old I would definitely back up everything, all the files and database first. If you do plan to upgrade I would suggest as Jamie said to upgrade to the latest .NET 3.5 version of mojoPortal before they make the change to .NET 4, then after they change it to .NET 4 upgrade to the .NET 4 version of mojoPortal with the same version number.
Hope that helps,