The Oracle database software (to_char function, RN number format element) does the following:
1990 - MCMXC
1999 - MCMXCIX
3999 - highest number translated to roman numerals, everything after that shows up as “overflow”
It seems to me that, where each answer is equally “right” given the ambiguities of ancient Latin, why not use the shortest notation? Thus, “MXM” for 1990, and IMM for 1999.
I remember M&M Mars trumpeting the “Year of M [and] M” in 2000.
It was part of Cartman’s speech about doing what’s right rather than what’s easiest.
Roman Numerals are like caligraphy. The whole point is the work involved else we’d just write 1990. The way it’s presented is the point, not the information involved.
XXXX for 40, for example, is found in monumental inscriptions, manuscripts, etc. It has nothing whatever to do with education levels. It’s simply a variant form.