We have waited till the Beta phase of .NET Framework 4.5 to finally decide on our platform story - what will be supported and what will be required.
It will continue to require .NET 4.0 as its runtime, but it will also run on .NET 4.5 (if you upgraded your machine). It supports (as compilation targets) 2.0 to 4.5 - if those frameworks are installed on your machine.
We are currently wrapping up feature development for 4.2, and we have plans for a further feature release 4.3. After that, the current plans are for servicing releases that update dependencies and/or fix bugs. Given those plans, expect 4.x releases for at least a year to come.
SharpDevelop 5 "Zimnitz"
This has been in development for some time already, and you have seen "offsprings" like ILSpy or Code Quality Analysis in SharpDevelop 4.2 to "prove" that our new NRefactory (*) is up to the job. Given that all IDE services need to be adapted to the new infrastructure, we will also take advantage of features in .NET 4.5, thus upping the requirements of SharpDevelop.
However, this means for a considerable part of our current user base to start planning for the future - because .NET 4.5 will likely not support XP (maybe not even Vista). Betas of SD5 will start showing up later this year.
Although the requirements change, the support of frameworks stays the same. (you will only have to install a newer OS on your development machine, deployment is unaffected)
* NRefactory 5 is developed together with the MonoDevelop team, and will support features such as semantic highlighting or more (sophisticated) refactorings.