#include<bits/stdc++.h>
using namespace std;
const long long M=2e5+10,MOD=1000000007;
typedef long long ll;
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    int t=1;
    cin>>t;
    while(t--){
     

    ll n,d,a,b;
    cin>>n>>d>>a>>b;
    ll res=min(n,d)*a;
    n-=d;
    if(n>0){
        res+=(n*b);
    }
    cout<<res<<"\n";
}




    
   
   return 0;
 
}