#include<bits/stdc++.h>
using namespace std;
/*#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>*/
typedef long long ll;
#define N 200001
#define lcm(a,b) ((a*b)/__gcd(a,b))
const ll mod=1e9+7;
        ll prime[N];
        vector<int>all_primes;
        ll prime_factors[N];
        void seive()
        {
            memset(prime,0,sizeof(prime));
            prime[0]=1;prime[1]=1;
            for(ll i=2;i*i<=N-1;i++)
            {
            if(prime[i]==0)
                {
                    for(ll j=i*i;j<N;j+=i)
                    {
                        prime[j]=1;
                    }
                }
            }
            for(int i=1;i<N;i++)
            {
                if(!prime[i])
                {
                    all_primes.push_back(i);
                }
            }
        }

        void spf()
        {
            for(ll i=1;i<N;i++)
            {
                prime_factors[i]=i;
            }

            for(ll i=2;i*i<=N-1;i++)
            {
                for(ll j=i*i;j<=N-1;j+=i)
                {
                    if(prime_factors[j]==j)
                        prime_factors[j]=i;
                }
            }
        }
//code starts here

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    int t;
    cin>>t;
    //t=1;
    while(t--)
    {
       ll n,d,a,b,ans;
       cin>>n>>d>>a>>b;
       if(d<=n)
      {
           ans=(d*a);
           ans+=(max((ll)0,(n-d))*b);
      }
       else
        ans=n*a;
       cout<<ans<<"\n";
    }
}