How to write Roman number from 4000 to Above in excel using ROMAN function.

From the Roman() function help document:

If number is greater than 3999, the #VALUE! error value is returned

So, you can not use the Roman() function for values "4000 and above".

The Roman() function stops at 3999 because that's the end of the "simple" roman numbers that can be represented with the basic letters (I, V, X, L, C, D, M), and adherring to the constraint that you can only repeat a letter at most 3 times in a row.  So there is some controversy over whether MMMM is valid as 4000.  Some people say yes, some no.  I sided with the yes on this question, and once you get to 3999 you use multiple M's to get larger than that.

There is evidence that Romans themselves worked around this by having a syntax for multiplying one of the base letters by 1000.  The two ways I have heard that they did this were putting a line over the top of the letter, or enclosing it in parens.  So both (M) and the image below could represent 1,000,000.So while the Romans() function limits itself to 3999, there are acceptable ways to represent larger numbers in Roman Numerals.

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Is there any unicode tool so that we can write roman number largr than 3999.

Not that I am aware of.  In HTML you can use overline, as in style="text-decoration:overline".  And things like Word support this as a modifier to characters, sort of like underline.  But I don't know of a single symbol code that has it for say an "M".

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