#include<bits/stdc++.h>
#define ll long long int
using namespace  std;
int main()
{ 
    ll t;
    cin>>t;
    while(t--){
        ll n,d;
        ll a,b;
        cin>>n>>d>>a>>b;
        ll dcost=d*a;
        ll rest=0;
        if(n>d){
            ll c=n-d;
            rest=c*b;
        }
        cout<<rest+dcost<<endl;
    }
}