#include<bits/stdc++.h>
using namespace std;
#define endl '\n'
#define ll long long int
#define vv vector<long long>
#define mpp map<long long,long long>
#define ss set<long long>
#define vp vector<pair<long long, long long>>
#define MOD 1000000007
#define YES cout<<"YES"<<endl
#define NO cout<<"NO"<<endl
#define all_v v.begin(),v.end()
#define all_a a.begin(),a.end()
#define all_n for(int i=0;i<n;i++)
void requisite(){
ios::sync_with_stdio(false);
cin.tie(nullptr);cout.tie(nullptr);
}
int main() {
requisite();
ll t;
cin>>t;
while (t--){
ll n,d;cin>>n>>d;
ll a,b;cin>>a>>b;
cout<<a*d+b*(n-d)<<endl;
}
return 0;
}