#include <stdio.h>
int main(){
int T;
scanf("%d",&T);

while (T--) {
        int N, D;  
        int A, B;
        scanf("%d %d", &N, &D);
        scanf("%d %d", &A, &B);

        long long total_cost;
        if (N <= D) {
        total_cost = (long long)N * A;
        }
        else {
        total_cost = (long long)D * A + (long long)(N - D) * B;
        }
        printf("%lld\n", total_cost);
}

    return 0;
}