calculate the number of occurrences of a "Letter" based on a percentage

Hi there,
I hope you can point me in the right direction. I have a 4 cells that contain a letter based on a score. I would like to total that range of cells using a weighted percentage so that it returns a letter value based on that percentage weighting. For instance:

Team Rating
Team1 R
Team1 G
Team1 A
Team1 R

Overall XXX

IF "G" is >=75% then insert "G"
IF "A" is >=50% then insert "A"
IF "R" is >=25% then insert "R"

I am struggling to figure out how I could do this as a formula, so would appreciate any pointers.

Many thanks
So If I was to do this manually I would put an "R" in the Overall cell.

That's great, thanks Glenn. It has worked perfectly!