// Bismillah hir-rahman nir-raheem
// Shakib Absar - (_rasbaris)

#include<bits/stdc++.h>
#include <tuple>
using namespace std;
#define   ll          long long
#define   ld          long double
#define   pb          push_back
#define   ff          first
#define   ss          second
#define   tc()        ll t; cin>>t; while(t--)
#define   endl        '\n'
#define   debug(x)    cout << " >> " << #x << " : " << x << endl
#define   yes         printf("Yes\n")
#define   no          printf("No\n")
#define   PI          acos(-1.0)
#define   Case(csNo)  cout <<"Case "<<csNo++<<": ";
#define   all(v)      (v).begin(),(v).end()
#define   print(v)    for(auto &i:v)cout<<i<<' ';
#define   _rasbaris_  ios_base :: sync_with_stdio(0);cin.tie(0);cout.tie(0);
// cout << "Case "<<q<<": ";
using namespace std;
const ll M = 1e9;

// 4 direction
int dx[] = {1, -1, 0, 0};
int dy[] = {0, 0, 1, -1};
int main()
{
    _rasbaris_

    tc()
    {



    ll n,d,a,b;
    cin >> n >> d;
    cin >> a >> b ;
    if(b<a)cout <<(d*a)+(n-d)*b << endl;
    else cout <<(d*b)+(n-d)*a << endl;


    }
    return 0;
}