#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
using namespace std;
#define ll long long 
ll mo(){
   char ch=getchar(),last=' ';
   while(ch<'0' || ch>'9')last=ch,ch=getchar();
   ll ans=0;
   while(ch>='0' && ch<='9')ans=ans*10+ch-'0',ch=getchar();
   if(last=='-')ans=-ans;
   return ans;
}
void solved(){
     int n,m,p,q;
     scanf("%d%d%d%d",&n,&m,&p,&q);
     if(p*m<=q)printf("%d\n",n*p);
     else{
          int t1,t2=0;
          t1=n/m*q+n%m*p;
          t2=n/m;
          if(n%m)++t2;
          t2*=q;
          t1=min(t1,t2);
          printf("%d\n",t1);
     }
}
int main(){
    int T;
    scanf("%d",&T);
    while(T--)solved();
    return 0;
}