Sometimes pictures are worth 1000 words. Here's what I did:
1) Created an Excel spreadsheet with one column holding a bunch of rows with a, b, c, and d randomly distributed.
2) Inserted a pivot table
2a) Selected the column with my a-d values as the range to pivot
2b) Added the pivot table to the existing worksheet
3) A window opens to the right with Pivot Table options. Dragged the "a" (representing my values) from the "Choose Fields" window to the "Column Labels" and the "Sum Values" window.
Once I did this, the pivot table updated to show the counts
Hope this helps!