#include "stack"
#include "stdio.h"
#include "iostream"
#include "math.h"
#include "string.h"
#include "algorithm"
#include "queue"
#include "string"
#define LL long long
#define inf 0x7ffffff
#define pi 3.1415926535897932384626433832795028841971
using namespace std;
int main()
{
    int t,n,m,p,q,d,sum;
    double k;
    scanf("%d",&t);
    while(t--)
    {
        scanf("%d%d%d%d",&n,&m,&p,&q);
        k=q*1.0/m;
        if(p<=k)
            printf("%d\n",p*n);
        else
        {
            d=n/m;
            sum=d*q;
            d=n-d*m;
            if(q<=d*p)
                sum+=q;
            else
                sum+=(d*p);
            printf("%d\n",sum);
        }
    }
    return 0;
}