Tim recently addressed this exact argument with "my books are not for everyone". He's not saying everyone on Earth should have a 4 hour work week.
That isn't a counter-argument to the criticism of the 4-Hour Work Week book. It feels like a cop-out on the part of Ferris.
In the 4-Hour Work Week, Ferris offers the advice that outsourcing tedious or menial tasks to economically poor countries allows you to reach your 4-hour work goal (the presumption being that you reside in a economically rich country that allows you to outsource to a cheaper country).
Disclaimer: I'm in the camp of readers who dislike the 4-Hour Work Week. Not just for the central premise of the book (outsource cheaply), but also for the excessively wordy style of writing that pads out the book.
The plebs should work 60 hours to support the lifestyle of the wealthy elites.
In all seriousness though it's clear his book is aimed at people that have a chance of creating enough wealth to escape wage slavery. For many people in the world this is simply not an option - I think that is the point the author is trying to make?