TAXIFARE Solution

import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {

	public static void main(String[] args) throws Exception {
		BufferedReader r =new BufferedReader(new InputStreamReader(System.in));


		long t=Long.parseLong(r.readLine());
		while(t-->0)
		{
		String[] value=r.readLine().split(" ");
			long first=Long.parseLong(value[0]);
			long M=Long.parseLong(value[1]);
			long N=Long.parseLong(value[2]);
			long dist=Long.parseLong(value[3]);

			System.out.println(Math.abs(first*M+((dist-first)*N)));

		}
	}

}