Convert in either direction between ordinary numbers and Roman numerals, with a breakdown showing which symbol contributes what. Handy for dates on buildings, film copyright lines, clock faces and homework.
A smaller numeral before a larger one is subtracted: IV is 4 and CM is 900. Only I, X and C are used this way, and only before the next two larger symbols, which is why 99 is XCIX rather than IC.
Standard Roman numerals have no symbol above M for 1000, so 4000 would need MMMM. Larger numbers historically used a bar over a numeral to multiply it by 1000, but that is not part of the standard set.
No. The system has no zero and no negative numbers, which is one reason arithmetic with it is so awkward compared to positional notation.