4.0a Beta is almost exactly the same as 3.x, except i think they fixed some bugs, added support for things like burn-proof, newer burners, and so on. I've never had any problems with it.
CDRWIN 5 is AFAIK not made by the same company that did CDRWIN 3.x. What happened is I think Goldenhawk used someone else's CLI burning program and just built a gui around it. They got sued, and after that, I think they had to redesign their burning program. I think some other company bought the rights or something, and released a 5.0. 4.0a beta was made right before they got sued.
Hmm...now that I think about it, I really don't know wtf I'm talking about, just spouting rumours I've heard on other forums. One thing is for sure though, CDRWIN 5.0 is a load of shit.
