#include<cstdio> #include<cstring> using namespace std; int main() { int t,n,m,p,q,sum; scanf("%d",&t); while (t--) { scanf("%d%d%d%d",&n,&m,&p,&q); sum=n*p; while (n>0) { if (n>m) { if (q<m*p) sum=sum-m*p+q; } else { if (q<n*p) sum=sum-n*p+q; } n-=m; } printf("%d\n",sum); } return 0; }