Starting with LR4, Adobe uses a new RAW processor(2012 ver). The RAW images are noticibly better when processed with the new version. If ISO's are high, you need it. NR is also greatly improved.
I've used every version heavily on several PC's and never had crashing issues, its likely a computer problem if its crashing.
Version 5 added some useful features beyond version 4, both have lens correction that will correct distortions for various lenses. They also have a photobook feature and ver 4 will import videos, but link to a external player while ver 5 will play video. You can take a typical video frame and make color and other corrections and then copy the corrections to the entire clip. The improvement can be dramatic and it jst takes seconds.
Starting with ver 4, you can soft proof your images to correct out of gamut photos so that your print will match what you see on the screen. Its not a matter of a calibrated monitor, out of gamut colors print differently.
I'm sure there is more.