#include<bits/stdc++.h>
#define ll long long int
using namespace  std;
int main()
{
    ll t;
    cin>>t;
    while(t--){
        ll n,d;
        cin>>n>>d;
        ll a,b,mult,mult1,sub;
        cin>>a>>b;
        mult=d*a;
        sub=abs(n-d);
        mult1=sub*b;
        cout<<mult+mult1<<endl;
    }
}