Run Font2DTest on RH EL 4, most of fonts do not show difference between Serif and SansSerif, neither between Dialog or DialogInput. More specificly, with some examples:
Font Range Shown as
--------------------------------------------------------------------------------
Serif Basic Latin OK
Serif/SansSerif Hebrew SansSerif
Serif/SansSerif Devanagari SansSerif
Serif/SansSerif Thai SansSerif
Serif/SansSerif Hiragana Serif
Serif/SansSerif Katakana Serif
Serif/SansSerif CJK Unified Ideogrpahs Serif

This seems to be the case since Mustang b01, didin't try with Tiger builds.
Where in RH AS 3, it seems better.

-----------------------------------------------------------------------------------------
I went back to the two RH systems and checked, and I think what happened was in RH4 system I had locale set to zh_TW when log in, and in RH3 system, I had it set to ja, so when I did testing on both system, I noted it was much better on Rh3, and forgot that two systems having locale set differently, shouldn't compare that way. So what I see from the testing is expected behavior.

Closing this as a duplicate of RFE 5004467.