Which XG Mobile?

I have a Z1E Ally and it is my only PC. I’ve been debating picking up an XG Mobile for it. I have the option of the 7850 for $500 or the 4090 for $1100. Is the 4090 worth double the price or should I save some cash and get the 7850? I have no desire to get a laptop or desktop so don’t bother suggesting that. Thanks.