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

int main()
{
    /*#ifndef ONLINE_JUDGE
        freopen("in.cpp","r",stdin);
        freopen("out.cpp","w",stdout);
    #endif // ONLINE_JUDGE*/
    int n,m,p,q;
    int t;
    while(scanf("%d",&t)!=EOF)
    {
        while(t--)
        {
            scanf("%d %d %d %d",&n,&m,&p,&q);
            int cost1=n*p;
            int cost2=n/m*q;
            if(n%m*p<q)
            {
                cost2+=n%m*p;
            }
            else
            {
                cost2+=q;
            }
            printf("%d\n",min(cost1,cost2));
        }

    }

    return 0;
}