#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;
#define pb push_back

void judge()
{
  #ifndef ONLINE_JUDGE
  freopen("error.txt", "w", stderr);
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
  #endif
 
}

const ll N=2e5 +5;

void soln(){
	int n,d;
	cin>>n>>d;
	int a,b;
	cin>>a>>b;
	ll ans;
	if(n>=d){
		 ans=(d*a)+((n-d)*b);
	}
	else
		 ans=n*a;
	
	cout<<ans<<endl;
}

int main() {
	//judge();
    int t;
    cin >> t;

    while (t--) soln();
}