Like most software programmes VideoStudio has evolved through the years - it presumably started way way back with VideoStudio 1 but I am not sure that there is any one left on here that remembers that, I started with version 3 - from then on new versions were issued , each version having a number of advances and additions over the previous versions. In the early days new releases were quite far apart but it seems to have become the norm now for a new version around about once a year, but I guess it would depend on whether Corel have made enough changes to justify a new release.
Versions 8 and 9 are quite old now (in software terms) and do not contain many of the features of the latest versions - but for many people they are still usable and do what they want - so although not officially supported by Corel in that they will not provide technical back up for users of those versions many of us on here used them and so try to help them if they have problems.
One of the other problems of course is that Microsoft keep changing their Windows systems so new versions of VideoStudio have to be released to run on the new systems - for instance versions 8 and 9 will not perform correctly on Windows 7 and probably have no hope on Windows 8.
So each version is an improvement on previous versions and contains features and options which previous versions did not have - I doubt if there is a list anywhere showing the difference between all versions back to the very beginning - but there is a list here :- http://www.corel.com/content/pdf/vsx5/v ... rix_en.pdf
which shows the differences in the last four versions.
your X4 (version 14) is relatively new and will work well on Windows 7 so unless there is something specific you want in X5 (version 15) then there is no need to worry about upgrading - many people jump at least one and often two versions before updating.