It makes sense to upgrade if you have trouble with InCD 3.x. In general InCD 4 works better with new devices, InCD 3.x better with old devices.
Regarding stability, the data from Microsoft about crash reports show a lot of problems with InCD 3.x, and much less with InCD 4. If we want to be pesimistic, this could although just mean that the amount of installed InCD 3.x is simply much higher.
(Additionally the problems reported by Microsoft to us on InCD 4 are all been fixed as soon as possible. Most of them where fixed in the recent 220.127.116.11 release.)
InCD Project Leader
Im Stoeckmaedle 18
fax: +49 (0)7248 928 299