copy/paste a dynamic sheet/cell formula using INDIRECT, cell not incrementing

Hi folks,

Im using a basic INDIRECT formula that references a cell on another sheet, using a a drop down list for the prefix of the sheet name. I can get the formula to work, but then Im wanting to copy that formula incrementing the cell address, and Im having no luck. Any help appreciated!

first row formula is

=INDIRECT(M4&"_Sheet!N4") *works for row 4*

when I cut/paste the formula to the next row, I get

=INDIRECT(M5&"_Sheet!N4"), BUT I need N4 to also increment to N5 so the paste result is:
=INDIRECT(M5&"_Sheet!N5")

I think I need to get the Nx outside the quotes, but I havent found anything that works....

TIA

Randall