"after DirectX 3 was released, Microsoft began developing versions 4 and 5 at the same time. Version 4 was to be a shorter-term release with small features, whereas version 5 would be a more substantial release. The lack of interest from game developers in the features stated for DirectX 4 resulted in it being shelved, and the corpus of documents that already distinguished the two new versions resulted in Microsoft choosing to not re-use version 4 to describe features intended for version 5."

https://en.wikipedia.org/wiki/DirectX#Version_history