For a very high-level overview that ignores many of the minor books and PDFs, Wikipedia has a good list of GURPS books. The impression that it gives is that 4th edition has a much reduced set of supplements, with much material that was in disparate books of various types in 3rd edition being amalgamated into single-topic books in 4th edition. There are also clearly far, far fewer "Powered by GURPS" titles published directly by SJG for 4th edition.
That article should be enough for most people. But there is more to the supplement situation in those editions than just what appears in the Wikipedia article. Someone who wanted a more detailed view of the situation would have to do some research to figure out what Wikipedia has left out as insignificant.
When I have such research questions, I consult the database at RPGGeek. They have an exhaustive listing of roleplaying items, broken down by game family and system, and categorised into "Sourcebooks", "Core Rules", etc.
For GURPS 3rd edition, scrolling down to the list of items and then selecting Sourcebooks From the Category drop-down results in 14 pages of RPG books. At 10 items per page, that's approximately 140 sourcebooks.
For GURPS 4th edition, doing the same results in 11 pages, or something in the neighbourhood of 110 sourcebooks.
This is only a very rough, first-pass estimate however and further research would yield a more detailed answer. Each of those include many very minor sourcebooks in the form of PDFs only a handful of pages in length, which I wouldn't personally count as the equal of the meatier books that run to hundreds of pages. Depending on your specific needs and criteria, you may agree or disagree with that. That rough estimate also doesn't include sourcebooks that are also campaign supplements, or supplements categorised as replacement rules systems, as those are under the Campaign Setting and Alternate Rules categories.
Without spending quite a bit of time and knowing in detail the requirements, a rough first pass is the best you can get from someone else. To really get a good picture of the state of the editions' supplements that you think "count", it's really necessary to go digging through the database yourself (or be fortunate enough to have a personal research assistant to handle the drudgery). The best I can do in this answer is point the way: RPGGeek's database is a valuable resource, and spending some quality research time with it can answer a lot of questions about the state of the two editions and how they compare to each other.