Yeah you have residues left over
Umm .. deleting the folder could fuck your system ... if it was outside your windows system directory, I would just say yeah delete the folder and erase the entries from your registry. But things that install in the Windows system folder are a bit more complicated I am afraid ... they 'tie' themselves into the system in strange ways sometimes. Deleting the folder could leave your system unstable.
There are various tools for getting rid of the previous .NET installations if they don't uninstall properly, but I dont think there is anything available for the .NET 4 beta yet. The best advice I can give is to follow that link I put below and follow the procedure on there and see if it works. If not ... ask on MSDN for some help, because you are going to need the caliber of assistance that they can offer ... its out of my league I am afraid.
Be polite with them, do everything they ask of you, and if you don't understand something ask them to explain it to you so you do. If you do that they will bend over backwards to help you. But dont follow their advice and they will just leave you to it.
For the record ... when it comes to beta software of things like operating systems and runtime engines etc ... you should only ever install them on a testing machine that is not actually important for anything else, as having to delete everything , reformat and reinstall everything right from the beginning is a likely scenario that might happen more than once during the beta phase.
Best of luck .... I feel for you
