/* 
*   @author:- SYEDA SADIATUL JANNAT TUSHI
*   date & time :2024-10-24 22:19:38
*   BANGLADESH , SYLHET. 
*/ 

#include <bits/stdc++.h>
using namespace std;

#define endl '\n'
#define fi first
#define se second
#define ll long long
#define ld long double
#define pb push_back
#define mp make_pair
#define vi vector<ll>
#define maxn 500005
#define mod 1000000007
#define inf 1000000007
#define pii pair<ll,ll>
#define vii vector<pii>
#define sz(x) (int)x.size()
#define all(x) x.begin(), x.end()
#define lcm(a,b) ((a*b)/__gcd(a,b)) 
#define srt(v) sort(v.begin(),v.end())
#define rsrt(v) sort(v.rbegin(),v.rend())
#define rep(i, a, b) for(int i = (a); i < (b); i++)
#define per(i, a, b) for(int i = (a); i > (b); i--)
#define rev_str(str) reverse(str.begin(),str.end());
#define print(v) for(auto e:v) cout<<e<<" "; cout<<endl;
#define printp(v) for(auto e:v) cout<<e.first<<" "<<e.second<<endl;
#define fast ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);

void who(){
    ll n,m;cin>>n>>m;
    ll a,b;cin>>a>>b;
    if(m>n){
        ll what = n*a;
        cout<<what<<endl;
        return;
    }
    ll what = (a*m);
    ll ki = (n-m)*b;
    cout<<ki+what<<endl;
}

int main(){
    fast
    ll t=1;cin>>t;
    while (t--)
    {
        who();
    }
    return 0;
}