/* SRIJON
SINGHA
SAMANTA */
#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef long double ld;
#define nl "\n"
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
#define yes cout << "YES" << nl
#define no cout << "NO" << nl
#define fi first
#define sec second
#define pb push_back
#define ppb pop_back
#define pf push_front
#define ppf pop_front
#define FIO \
ios_base::sync_with_stdio(false); \
cin.tie(NULL); \
cout.tie(NULL);
const int mod = 1e9 + 7;
int32_t main()
{
FIO
ll t; cin>>t;
ll tt=t;
while (tt--)
{
ll n,d; cin>>n>>d;
ll a,b; cin>>a>>b;
ll ans=min(n,d)*a;
if(d<n) ans+=(n-d)*b;
cout<<ans<<nl;
}
return 0;
}