This can be done w/o VBA. It assumes the letter headings are in row 1.
If that isn't true, put them there temporarily. To the right or below
the data put in one cell the formula =A$1 & "-" &
IF(A2<4,"Low",IF(A2<8,"Med","Hi")) Copy that cell and paste to a
block of cells the same size as your data, with the copied cell being
1st row, 1st column. Copy and Paste Special | Values the entire block
over the original data.

Hth,
Merjet