#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
void solve(){
  int l,r,a,b;
  cin>>l>>r>>a>>b;
  if (a>=0) cout<<a*r+b<<endl;
  else cout<<a*l+b<<endl;
}
int main(){
  int t;
  cin>>t;
  rep(i,t) solve();
}