#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define bug(a) cout<< #a << " : " << a <<endl;
#define bug2(a, b) cout<< #a << " : " << a  << " " << #b << " : " << b << endl;

void solve(int cs){
    int n,m;cin>>n>>m;
    int a,b; cin >> a >> b;
    ll ans = 0;
    ans += a * 1ll * m;
    n -= min(m,n);
    ans += b * 1ll * n;
    cout << ans << '\n';
}


int main(){
    ios_base::sync_with_stdio(false);cin.tie(0);
    cout.tie(0);

    int tc = 1;
    cin >> tc;

    for(int cs = 1; cs <= tc; cs++){
        solve(cs);
    }
}