Grading an array using 4 letter grades with additional Criteria

I have a large spreadsheet with multiple categories with 4 separate letter grades and an "NA" for non applicable. I need a formula which can provide the following criteria based upon an array of cells:

5 Total "Grades"
-FD: FD is the end result if ANY cell within the Range possesses "FD".
-G: The end result can only be "G" if all criteria in a range are also "G".
-A: A is a final grade if all cells are "G" except ONE "A". IF there are more than 2x "A", the End Result MUST be "FD"
-R: R is the final grade if all cells are "G" except ONE "R". IF there are more than ONEx "R", the End Result MUST be "FD". (note: if one of the grades on the previous cell is an "A" instead of a "G" with an A, the result should also be "FD".
-NA is simply NOT counted (nor does it influence any of the above "grades"), unless ALL cells are NA, in which case the End Result is "NA".

This seems convoluted. I've tried a nested countif function which encompasses most of the criteria, EXCEPT it does not allow a transition to an "FD" end result if more than 2 x A or 1x R are present (it simply says "A" in the former and "R" in the latter). Note the "4" in the last and second to last denotes the total range size.

=IF(COUNTIF(B4:B7,"FD")>0,"FD",IF(COUNTIF(B4:B7,"R")>0,"R",IF(COUNTIF(B4:B7,"A")>0,"A",IF(COUNTIF(B4:B7,"NA")=4,"NA",IF(COUNTIF(B4:B7,"G")+COUNTIF(B4:B7,"NA")=4,"G")))))


Thanks!!!

Sorry, attached is the table (which is a much scaled down version... suffice to say there are more tables from which I have to "average' the categories).Sum of Tables.xlsx.