BobsRevenge: Maybe they want to put more work into them before releasing since they're bigger profile?
Probably this. Considering how the majority of players prefer SH2 and SH3 over SH4, they may just put in extra effort to fix the PC Ports of those two games.
It would also be weird for Konami to hand out the license for SH4, but not for SH2 and SH3, which were more beloved within the community and would thus create more money.
Not to mention that SH2 and SH3 are
older, even if just by one or two years. I can imagine that just the fact that it's later in the series may allow SH4 to run better, or be easier to update.
I recall seeing screenshots of people's graphic's cards struggling with the moving texture on Eileen's skin in the original SH4 PC port. So I can only imagine how much the moving textures of SH3's Otherworld walls would be an issue in the same case. Now granted, these issues were a few years ago (roughly around 2010-12), but even by then the games were already
old and probably struggling with some hardware.
Heck, SH2-4 are turning 20 years old in the next few years. Hardware and software have developed massively since then.
If they have the license, it probably takes them more time to update and patch the two older games.