The Roman numeral for 4 is IV. To write 4 in Roman numerals, we can use the method of breaking it into the difference of 5 and 1, which gives us V - I = IV. It is worth noting that while IV is the standard notation for 4 in Roman numerals, there are cases where IIII is used instead, particularly on the faces of clocks. This is due to a variety of historical and aesthetic reasons, such as the symmetry of IIII with VIII and the belief that IV was bad luck. However, in most contexts, including mathematics and formal writing, IV is the accepted and preferred notation for 4 in Roman numerals.