Agreed... the graphic stack was greatly improved.
You should be able to run Fusion 4 in a seperate folder... all the backend stuff should be self-contained... It gets instantiated when Fusion launches.
I haven't tested this, but it's an easy thing to try...
Install 1, drag it out of the applications folder, install the other.
Could you elabourate on 'does not support opengl or directx in an adequate way'?
(The graphics team put a ton of work into this release! They'd be saddened to hear you say that...)