/*#include <bits/stdc++.h>
using namespace std;
typedef pair<string,int> P;
char ans[50002];
string temp[115] = {"H","He","Li","Be","B","C","N","O","F","Ne","Na","Mg","Al","Si","P","S","Cl","Ar","K","Ca","Sc","Ti","V","Cr","Mn","Fe","Co","Ni","Cu","Zn","Ga","Ge","As","Se","Br","Kr",
    "Rb","Sr","Y","Zr","Nb","Mo","Tc","Ru","Rh","Pd","Ag","Cd","In","Sn","Sb","Te","I","Xe","Cs","Ba","Hf","Ta","W","Re","Os","Ir","Pt","Au","Hg","Tl","Pb","Bi","Po","At","Rn",
    "Fr","Ra","Rf","Db","Sg","Bh","Hs","Mt","Ds","Rg","Cn","Fl","Lv","La","Ce","Pr","Nd","Pm","Sm","Eu","Gd","Tb","Dy","Ho","Er","Tm","Yb","Lu",
    "Ac","Th","Pa","U","Np","Pu","Am","Cm","Bk","Cf","Es","Fm","Md","No","Lr"};
int main ()
{
    int T;
    scanf("%d",&T);getchar();
    while (T--)
    {
        scanf("%s",ans);
        queue<P> q;
        string t = "";
        t += ans[0];
        if (find(temp,temp + 114,t) != (temp + 114))q.push(P(t,0));
        t += ans[1];
        if (find(temp,temp + 114,t) != (temp + 114))q.push(P(t,0));
        bool flag = false;
        while (!q.empty())
        {
            P p = q.front();
            int index = p.second;
            if ()
            string s = "";
            if (p.first.size() == 1)
            {
                s += ans[index + 1];
                if (find(temp,temp + 114,s) != (temp + 114))q.push(P(s,index + 1));
                s += ans[index + 2];
                if (find(temp,temp + 114,s) != (temp + 114))q.push(P(s,index + 1));
                q.pop();
            }
            else
            {
                s += ans[index + 2];
                if (find(temp,temp + 114,s) != (temp + 114))q.push(P(s,index + 2));
                s += ans[index + 3];
                if (find(temp,temp + 114,s) != (temp + 114))q.push(P(s,index + 2));
                q.pop();
            }
        }
    }
    return 0;
}*/
#include <iostream>
#include <cstdio>
using namespace std;
int main ()
{
    int T;
    scanf("%d",&T);
    while (T--)
    {
        int n,m,p,q;
        scanf("%d%d%d%d",&n,&m,&p,&q);
        if (m * p <= q)printf("%d\n",n * p);
        else
        {
            int zu = n / m,sh = n - zu * m;
            if (sh == 0)printf("%d\n",zu * q);
            else
            {
                if (p * sh >= q)printf("%d\n",(zu + 1) * q);
                else printf("%d\n",zu * q + sh * p);
            }
        }
    }
    return 0;
}