Resident Evil 2 was the biggest game in the series by far. It was pretty much THE game until Ocarina of Time. When RE4 came out, it didn't have that same kind of hype or lustre of RE2. RE4 was big, but it was never the IT game like RE2 was in the day. RE4 is was like Skyward Sword or FF13-2, while RE2 was more like Breath of the Wild or FF7.
According to VG Chartz, RE2 ended up just under 6 million, while RE4 sold 3.6M on its PS2 launch, and then later 2.2M on the Wiimake. But if anyone is like me, then they bought both versions of the game, while it is unlikely any more than a very trivial number (like maybe 0.001%) bought multiple copies of RE2 on PSX. Plus the series was still new and fresh back then, it didn't have the strong legacy of past releases going for it.