Hey all. So is this something that someone can shed some light on.

Let's start with a simpler problem statement: "when N4, D491, and E491 all contain date values, does N4 fall between the other two (inclusive)?" For this, consider:
=IF(MEDIAN($N$4,D491,E491)=$N$4, M491, "na")

If that's not the intention, does N4 actually contain a date value, or a text string?

If N4 contains a string and if the dates to be compared are all the first of the month, then consider:
=IF(MEDIAN(DATEVALUE($N$4&" 1, 2018"),D491,E491)=DATEVALUE($N$4&" 1, 2018"),
M491,"na")

I'm not sure if this covers the problem intended in th original post. Other possibilities might be :
- The first of N4's month is compared with D491 and E491
- The entire month of N4 must be between the two dates D491 and E491
- something else.

Hopefully, the formulas above can be a starting point in these cases. Other functions than might be useful include EOMONTH(...) and DATE(...).

If more is needed, please be a lot more specific about the problem statement, what you tried, and how the result you got differed from what's needed.