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

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