I have purchased Lightroom 4.0, but I have not installed it.  I am using the Lightroom 4.1 RC, and will continue to do so until 4.1 is officially released.  Then I will install it because I'm not having any problems with the release candidate.  It seems to be quite stable.

Technically speaking, you can share the images as you suggest.  But I think it's recommended that you work on separate images.  I seem to recall that some adjustments carried over, although I may be wrong in that regard.  Since I switched to the 4.1 RC I have not used Lightroom 3.  When you run Lightroom 4 you are presented with an option to convert your current catalog.  The new version for Lightroom 4 is created in addition to what you already have.  Go ahead and try it the way you want.  Although I recall problems reported, I cannot say so for sure.

I have LR 4 RC1 and LR 3.6 on my computer.

I find that the 4 RC1 is still draining memory to the point of making the computer useless...and 1980's slow!!!

TO keep both version of LR on my computer I made External hard drive second copy of my LR 3 catalog with metadata and presets and a full set of photos for protection.

Also I'm not sure if this was important but instead of doing an upgrade...I installed a complete LR 4.

I did not import the copies files into LR 4  but the 4.9 info are the original copy ONE in LR 3.6....then I imported the external drive's set of 3.6 infor back into the 3.6 catalog to make sure nothing was touched by 4.0

I'm sure it's overkill some would say...the UPGRADE was supposed to keep both...but didn't.  Once imported into 4RC you can't see changes prior to that.

I'm happy to track to advances in 4 RC1...but feel safe that I can go back into 3.6 to do modification.