#include<iostream>
#include<cstdio>
#define LL long long
#define eps 1e-8
#define inf 0x3f3f3f3f
#define PI acos(-1.0)

using namespace std;

int main()
{
    int n,m,p,q,t;
    scanf("%d",&t);
    while(t--)
    {
        scanf("%d%d%d%d",&n,&m,&p,&q);
        if (p*m<=q) printf("%d\n",n*p);
        else
        {
            int u=n/m;
            int v=n-u*m;
            int ans1=u*q+v*p;
            int ans2=n*p;
            int ans3=(u+1)*q;
            printf("%d\n",min(ans1,min(ans2,ans3)));
        }
    }
    return 0;
}