#include<iostream>
#include<cmath> 
#include<cstdio>
#include<algorithm>
#include<ctime>
#define N 200005
#define INF 0x4fffffff
#define PI 3.14159265358979323846
#define MM 1000000007

#define rep(i,j,k) for(i=j;i<=k;i++)
#define rrep(i,j,k) for(i=j;i>=k;i--)

using namespace std;

int T,n,m,p,q,s,t;
int ans;

int main(){
    cin>>T;
    while(T--){
        scanf("%d%d%d%d",&n,&m,&p,&q);
        ans=p*n;
        t=n/m;
        s=n%m;
        ans=min(ans,t*q+s*p);
        ans=min(ans,(t+1)*q);
        printf("%d\n",ans);
    }
    return 0;
}