Alphanumeric nested if condition

Hello,

I worked on the following formula, however, I want one more condition to be checked. Currently it is validating if cell is blank, then checking if cell value=>31, then range of numbers between 0 to 35 to return 1,2,3 or 4.
=IF(F3=0,"Cell Blank",IF(F3="> 31 Days",1,LOOKUP(F3,{0,20,25,30,35},{"4","3","2","1"})))

I want one more validation, if cell value ="NA", then it should return 4

I am unable to get it right. Please help

Regards