I think 3999 is MMMCMXCIX. MMMCMCXIX would take 100 from 1000, but then add it back, which is not possible (otherwise it would be 4019, but it is written as CDXIX).
I think 3999 is MMMCMXCIX. MMMCMCXIX would take 100 from 1000, but then add it back, which is not possible (otherwise it would be 4019, but it is written as CDXIX).