How Do You Write 4000 In Roman Numbers?

Large numbers can be written in Roman numerals using an overbar to indicate multiplication by 1000, though the Romans may not have done it that way.
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IV = 4000

Many people might say that it's MMMM
but you can't write four consecutive letters in roman no.
If it was to be MMMM then why don't we write four as IIII
since we write four as IV
then it must be IV and compliment or a bar sign on top of V
that signalize as * 1000 so its four thousand