import java.util.Scanner;

public class Main {
    public static void main(String... args){
        Scanner sc=new Scanner(System.in);
        int t=sc.nextInt();
        for(int i=0;i<t;i++){
            int l=sc.nextInt();
            int r=sc.nextInt();
            int a=sc.nextInt();
            int b=sc.nextInt();
            if(a>0){
                System.out.println(a*r+b);
            }else{
                System.out.println(a*l+b);
            }
        }
    }
}