#define PROB Training
#include <cstdio>
#include <cstring>

#define f(x,y,z) for(int x = (y),__ = (z);x < __;++x)
#define g(x,y,z) for(int x = (y),__ = (z);x <= __;++x)
#define gd(x,y,z) for(int x = (y),__ = (z);x >= __;--x)
#define CLR(X) memset(X,0,sizeof(X))
#define SET(ARR,X) memset(ARR,X,sizeof(ARR))
#define R(X,Y) f(X,0,Y)
#define G(x,y) g(X,0,Y)
#define MAXN 1024


#ifdef WIN32
    #define LLD "%I64d"
    #define LLU "%I64u"
#else
    #define LLD "%lld"
    #define LLU "%llu"    
#endif

typedef long long LL;
typedef long double real;
int t;
int n,m,p,q,sz;
int main(){
    scanf("%d",&t);
    while (t--){
        scanf("%d%d%d%d",&n,&m,&p,&q);
        if (p * m <= q){
            printf("%d\n",n * p);
        }else {
            sz = n / m * q;
            if (n % m * p < q)sz += n % m * p;
            else sz += q;
            printf("%d\n",sz);
            // printf("%d\n",(n / m) * q + (n % m) * p);
        }
    }
    
    return 0;
}