#include<iostream>
using namespace std;
int main(){
    int T;
    cin>>T;
    while(T--){
        int N,D,A,B;
        cin>>N>>D;
        cin>>A>>B;
        int fee;
        if(N<=D){
            fee=D*A;
        }
        else {
        fee=(D*A)+((N-D)*B);
        }
        cout<<fee<<endl;
    }

return 0;
}