#include <stdio.h>

int main() {
    int T; 
    scanf("%d", &T);

    while (T--) {
        int N, D, A, B;
        scanf("%d %d", &N, &D);
        scanf("%d %d", &A, &B);
        
        int total_cost = 0;
        if (N <= D) {
            total_cost = N * A;
        } else{
            total_cost = D * A;
            total_cost += (N - D) * B;
        }

        printf("%d\n", total_cost);
    }

    return 0;
}