#include <stdio.h>
void main(void){
    int n = 0;
    scanf("%d",&n);
    int maxList[n] = {};
    for(int i = 0 ; i < n ; i++){
        int l,r,a,b;
        scanf("%d %d %d %d",&l,&r,&a,&b);
        if(a > 0 && l > r || a < 0 && l < r){
            maxList[i] = a*l+b;
        }else{
            maxList[i] = a*r+b;
        }
    }
    for(int i = 0 ; i < n ; i++){
        printf("%d\n",maxList[i]);
    }
}