//"তুই কে  - রাজাকার  আমি কে - রাজাকার কে বলেছে কে বলেছে - স্বৈরাচার স্বৈরাচার " [Our PM  said we are  "রাজাকার" ] ~ from protester_Kaminaaa 16.7.2024

/*
⣿⣿⣿⣿⡿⠟⠛⠋⠉⠉⠉⠉⠉⠛⠛⠻⠿⢿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿
⣿⣿⠟⠈⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⠀⠈⠙⠾⣿⣾⣿⣾⣿⣾⣿⣾⣿
⠋⡁⠀⠀⠀⠀⠀⢀⠔⠁⠀⠀⢀⠠⠐⠈⠁⠀⠀⠁⠀⠈⠻⢾⣿⣾⣿⣾⣟⣿
⠊⠀⠀⠀⠀⢀⠔⠃⠀⠀⠠⠈⠁⠀⠀⠀⠀⠀⠀⠆⠀⠀⠄⠀⠙⣾⣷⣿⢿⣿
⠀⠀⠀⠀⡠⠉⠀⠀⠀⠀⠠⢰⢀⠀⠀⠀⠀⠀⠀⢰⠀⠀⠈⡀⠀⠈⢿⣟⣿⣿
⠀⠀⢀⡜⣐⠃⠀⠀⠀⣠⠁⡄⠰⠀⠀⠀⠀⠀⠀⠐⠀⠀⠀⠰⠀⠀⠈⢿⣿⣿
⠀⢠⠆⢠⡃⠀⠀⠀⣔⠆⡘⡇⢘⠀⠀⠀⠀⠀⠀⠈⠀⠀⠀⠀⠀⠀⠀⠀⣿⣿
⢀⡆⠀⡼⢣⠀⢀⠌⢸⢠⠇⡇⢘⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠘⣿
⣼⣃⠀⠁⢸⢀⠎⠀⢸⠎⠀⢸⢸⡄⠀⠀⠀⠀⠀⠂⢀⠀⠀⠀⠀⠀⠀⠀⠀⣿
⠃⡏⠟⣷⣤⠁⠀⠀⠸⠀⠀⡾⢀⢇⠀⠀⠀⠀⠀⠄⠸⠀⠀⠀⠀⠄⠀⠀⠀⣿
⠀⠀⣀⣿⣿⣿⢦⠀⠀⠀⠀⡧⠋⠘⡄⠀⠀⠀⠀⡇⢸⠀⠀⠠⡘⠀⠀⠀⢠⣿
⠈⠀⢿⢗⡻⠃⠀⠀⠀⠀⠀⠀⠀⠀⠱⡀⠀⠀⢰⠁⡇⠀⠀⢨⠃⡄⢀⠀⣸⣿
⠀⠀⠀⠈⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣀⣱⠀⠀⡎⠸⠁⠀⢀⠞⡸⠀⡜⢠⣿⣿
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢠⣺⣿⣧⢰⣧⡁⡄⠀⡞⠰⠁⡸⣠⣿⣿⣿
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠠⡿⠏⣿⠟⢁⠾⢛⣧⢼⠁⠀⠀⢰⣿⡿⣷⣿
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⠉⠡⠄⠀⡠⣚⡷⠊⠀⠀⠀⣿⡿⣿⡿⣿
⡀⠀⠀⠀⠀⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⡠⠊⠁⢸⠁⠀⠀⠀⢰⣿⣿⡿⣿⣿
⠱⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⡠⠊⠀⠀⠀⡞⠀⠀⠀⠀⢸⣿⣷⣿⣿⣿    /*"kawaiiiiiii!!!!!!!!!!!"*\
⠀⠙⢦⣀⠀⠀⠀⠀⠀⢀⣀⣠⠖⠁⠀⠀⠀⠀⠀⡇⠀⠀⠀⠀⣸⣿⣾⡿⣷⣿
⠀⠀⠀⠀⠉⠉⣩⡞⠉⠁⠀⢸⡄⠀⠀⠀⠀⠀⢰⠇⠀⠀⠀⠀⣿⣿⣷⣿⣿⣿
⡆⠀⠀⣀⡠⠞⠁⣧⢤⣀⣀⣀⡇⠀⠀⠀⠀⠀⣸⠀⠀⠀⠀⠀⣿⣷⣿⣷⣿⣿
⣿⣷⠊⠁⠀⠀⡰⠁⠀⠀⠀⠀⣹⠶⢦⡀⠀⠀⡇⠀⠀⠀⠀⠀⢸⣿⣷⣿⣷⣿
⣿⢿⠀⠀⠀⡔⠁⠀⠀⠀⠀⠀⠀⠀⠀⠈⠳⡄⡇⠀⠀⠀⠀⠀⠈⣿⣾⣷⣿⣿
⠋⠈⠀⢀⠜⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⡀⠀⠈⣧⠀⠀⠀⠀⠀⠀⠻⣿⣽⣾⣿
⢀⡄⡠⠊⠀⠀⠀⠀⠀⠀⠀⠀⠀⡀⠁⠀⠀⠀⣸⠀⠀⠀⠀⠀⠀⠀⣿⣿⣻⣿
⣿⠋⠀⠀⠀⠀⠀⠀⠀⠀⢰⠀⠐⠀⠀⠀⠀⣀⡿⠀⠀⠀⠀⠀⠀⠀⢹⣿⣻⣿
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⡼⠀⠀⠀⠀⠀⢀⣃⡇⠀⠲⡀⠀⠀⠀⠀⠈⣿⡿⣿
⣀⠤⠤⠤⡀⠀⠀⠀⠀⡴⠃⠀⠀⠀⠀⠀⢬⠞⡇⠀⠀⠇⠀⠀⠀⠀⠀⣿⣿⣿
⡁⢀⠀⠀⡇⠀⠀⠀⡼⠁⠀⠀⠀⠀⠀⣸⠁⠀⠇⠀⠀⡇⠀⠀⠀⠀⠀⣿⣿⣿
⠔⠃⠀⠀⡇⠀⠀⡼⠁⠀⠀⠀⠀⠀⢀⡇⠀⠀⡃⠀⠀⠙⢄⠀⠀⠀⠀⣿⣷⣿
⠒⠊⠀⠀⢸⠀⣸⠃⠀⠀⠀⠀⠀⠀⡞⠀⠀⠀⢅⠀⠀⡂⠸⡄⠀⠀⠀⣿⣟⣿
⠓⠀⠉⠀⢸⣰⠃⠀⠀⠀⠀⠀⠀⡜⡆⠀⠀⠀⢸⠀⠀⡇⢀⠇⠀⠀⠀⣿⣿⣿
⠉⠁⠀⢠⠞⠀⠀⠀⠀⠀⠀⠀⣰⠁⡇⠀⠀⠀⡇⠀⠀⡇⢸⠀⠀⠀⠀⣿⣷⣿
⡀⠀⢀⢿⣥⡤⠤⠤⠤⣀⣀⢠⠇⠀⢸⠀⠀⢰⠁⠀⢨⠀⢸⠀⠀⠀ ⣿⣟⣿
*/
/*
/* writer  : chainsa_min *\
   time    :  20:49
   date    :  5/9/2024

*/
#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;
// hellow it's me .. "me ?" you beloved bastard_chao_min  :] and tnx to you  for reading my code
#define ll                 long long
#define maxi               1e17
#define MOD                1e9 + 7
#define pie                3.14159265358979323846264338327
#define nissan             for (i=0; i<n; i++)
#define skyline            for(i=1;i<=n;i++)
#define AE86               for(i=0;i<n-1;i++)
#define Rx7                for(i=1;i<n;i++)  //Mazda RX-7 Savana.
#define in                 cin>>n
#define on                 string s
#define up                 cin >> s
#define be(v)               v.begin(), v.end()//v(n)
#define sbe(v);              sort( v.begin(), v.end())//v(n)
/* ´´´´´´´´´´´´´´´´´´´´´´´´´´´´´´´´HELLO´´´´´´´´´´´´´´´´´´´´´´´´´´´´´´´ */
/* _________________________It's Me DEADPOOL!! (>‿◠)✌´ _________________________*/
#define mii                map<int,int>v;
#define mcl                 map<char,ll>
#define msl                map<string,ll>
#define emb                emplace_back
#define mpi                make_pair
#define seti               pair<ll,ll>
#define pub                push_back
#define vei                vector<int>
#define ves                vector<string>
#define vec                vector<char>
#define vpl                vector<seti>
#define deq                deque<ll>
#define maxv(a,b)          (a)=max((a),(b)) // set a to the maximum of a and b
#define minv(a,b)          (a)=min((a),(b));
#define debv(v)            for(auto it : v) cout << it << " ";
#define deb(x)             cout << #x << " = " << x << " ";
#define vel                vector<ll>
#define bv2                bitset<14> b(v[i])//2^n = x n = 16 32 64 ......
#define b32                bitset<32>
#define chifuyu            iota(v.begin(), v.end(), 1)
#define mustano            accumulate(v.begin(), v.end(), 0LL)
#define sei                set<ll>
#define mappa              map<ll,ll>
#define sj7                ara[n]
#define supra              ara[i]
#define matsuno            nissan { cin >> ara[i] ;}
#define dazai              nissan {cin >> v[i];}
#define dbg(a)               cerr << __LINE__ << ": " << #a << " = " << a << '\n'
#define optimus_prime      vector<int>::iterator it
#define flash              cout << *it << endlll // if(*it==n)  *it = x (x replacing n && sit on this place)
#define nsx_r              cin >> ara[i]
#define evo                n = s.size();
#define ash                  ll j ,  i,  n , m ,sums , a , b, c,d,  val_stor , val_stor1 , val_stro2 , val_stor3 , count_kor = 0 , k , sum = 0 , x , y, z , count_kor_1 = 0
#define gtr                '\n'
// double sub_x = pow(x1 - x2, 2);
#define chainsaw           cout << "YES" <<gtr
#define vapub              nissan { cin >> x; v.pub(x);}
#define man                cout << "NO"<<gtr
#define se                 ' '
#define se2                " "
#define tithi             s.begin(), s.end() // dont think she my gf -;( she is mine own little sister -:}
#define mitsuha_wifu      ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL)
#define kyouka_chan       freopen("input.txt", "r", stdin);freopen("output.txt", "w", stdout);
#define folafol(x)         cout << x << gtr ;
#define folafol_1(x,y) ;      cout << x << se2 << y ;
#define she(a,b,i)        for (ll i = a; i < b; i++)
#define her(a,b,i)        for (ll i = a; i <=b; i++)
#define bjt(a,b,i)        for(ll i = a ; i>=b ; i-- )
#define ss                second
#define println(x)         cout << x << gtr ;
#define folafol_2(x,y)       cout << x << " " << y ;  cout << gtr ;
#define sayonara          return 0
#define lancer_            ll  guts ; cin >> guts ; while(guts--)
#define hell_               #ifndef  ONLINE_JUDGE
#define _sign              kyouka_chan ; #endif
//---------------------------start---------------------------------
// const int MOD = 1e9 + 7;
//  double average = static_cast<double>(sum) / n;
//  sort(a(v), greater<ll>()); means  reverse(a(v)) ;


bool comp_are (seti a , seti b) {
  if(a.first==b.first){
    return a.second<b.second ;
  }
  else
 return  a.first > b.first;
}
bool comps (seti a , seti b) {
  if((a.first == b.first))
 return (a.second > b.second);
 else
  return (a.first<b.first) ;
}
// for 2 pointer l = 0 r= n-1 \

ll tringle(ll a , ll b, ll c ) {
    if((a+b)>c && (b+c)>a  &&   (c+a)>b)
    {
       return  1 ;

    }
    else return 0 ;
}
ll  segment (ll a , ll b, ll c ) {
    if((a+b)==c  || (b+c)==a ||  (c+a)==b)
    {
       return  1 ;

    }
    else return 0 ;
}
#define outs(x)               for(auto a:x) cout << a << " "; cout << endl


ll N = 500;

int main (){
   mitsuha_wifu ;
 //bool flag = 0 ; // true
  #ifndef  ONLINE_JUDGE
    kyouka_chan ;
   #endif
lancer_
 {
ash  , zero_cnt = 0 , one_cnt = 0 , e , f , val , val1  ;
bool flag = 0 ;
cin >> n >> d; 
cin >> a >> b ; 
if (n <= d) {
    sum = n * a;
        } 
        else {
            
            
            sum = (d * a) + ((n - d) * b);
        }
        
        cout << sum << endl; 

 }
return 0 ; 
}