#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
using namespace std;

int main()
{
    int T;
    scanf("%d",&T);
    while(T--)
    {
        int n,m,p,q;
        scanf("%d%d%d%d",&n,&m,&p,&q);
        double Dan=1.0*q/m;//套装单价
        if(n<=m)
        {
            if(n*p>=q) printf("%d\n",q);
            else printf("%d\n",n*p);
        }
        else
        {
            if(1.0*p<=Dan) printf("%d\n",n*p);
            else
            {
                int ans=0;
                ans=n/m*q;
                ans=ans+min((n%m)*p,q);
                printf("%d\n",ans);
            }
        }
    }
    return 0;
}