
Font formats are totally same in thps3, thps4 and thug1 (let's say FNT1), though there are minor changes in thug2 (FNT2) and it change a lot in THAW (FNT3?).
they can be named as small.fnt, smallfnt.dat, smallfnt.xbx or anything else but they are actually same files, so all that you have to do is to copy it in fonts folder and replace the existing one.
Here you can find small library of FNT1 fonts: http://devilclanthps3.weebly.com/fonts.html
*the only difference may appear in special symbols*
FNT Tool: viewtopic.php?f=8&t=229
here are some thps4 screens using thug1 and thug2 fonts:

