#include<cstdio>
#include<algorithm>
using namespace std;
int main()
{
    int T,n,m,p,q;
    int num;
    
    scanf("%d",&T);
    while(T--)
    {
        scanf("%d%d%d%d",&n,&m,&p,&q);
        if(p*m<=q)
        {
            num=n*p;
            printf("%d\n",num);
        }
        
        else{
            num=(n/m)*q;
            num+=min(q,(n%m)*p);
            printf("%d\n",num);
            
        }
        
        
    }
 }