Convert numbers to Roman numerals and Roman numerals back to numbers, from 1 to 3,999.
I = 1, V = 5, X = 10, L = 50, C = 100, D = 500, M = 1,000. Symbols are added, except that a smaller one before a larger one is subtracted: IV = 4, IX = 9, XL = 40, CM = 900.
2026 is MMXXVI; 1994 is MCMXCIV.
Standard numerals have no symbol above M, so 4,000 would need four Ms, which isn't standard.