#include<bits/stdc++.h>
#define ll unsigned long long
using namespace std;

void solution(){
     ll n,d;
     cin>>n>>d;
     ll c = n-d;
     ll a,b;
     cin>>a>>b;
     ll M,m;
     if(a<b){
         M = d*b;
         m=c*a;
     }
      M=d*a;
      m=c*b;
     ll sum=M+m;
     cout<<sum<<endl;
}
int main(){
    ll t;
    cin>>t;
    while(t--){
        solution();
    }
    return 0;
}