#include<bits/stdc++.h>
using namespace std;
#define optimize() ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define endl '\n'
#define ll long long int
int main()
{
    optimize();
    int t;
    cin >> t;
    while(t--){
    int a,b,c,d;
    cin >> a>>b>>c>>d;
   if(c>d) cout<<(b*c)+(abs(a-b)*d)<<endl;
   else cout<<(b*d)+(abs(a-b)*d)<<endl;

    }
 }