#include<stdio.h>
#include<algorithm>
using namespace std;
int i,j,k,l,s,m,n,test,p,q,ans;
int main() {
    scanf("%d",&test);
    while (test--) {
        scanf("%d%d%d%d",&n,&m,&p,&q);
        if (m*p<q) q=m*p;
        ans=n/m*q+(n%m)*p;
        ans=min(ans,(n/m+1)*q);
        printf("%d\n",ans);
    }
}