awk -F, 'NR>1 {a[$3]+=$4} END {for (k in a) print k, a[k]}' data.csv

This awk one-liner implements a group-by sum: the array a indexes by field 3's value and accumulates field 4 into it, then the END loop prints each key with its total. It is the classic awk pivot idiom, but hand-rolled. For readability and extra statistics, prefer datamash -g or mlr stats1, which do the same with clearer syntax. The output order of keys is unspecified.