Caveat: I didn't read the book and I'm not an expert. For me it doesn't make any sense. 4 hours a week is simply not enough for your body. According to some study even 30 min. a day is not enough. Just sitting 8+ hours a day has deleterious effects that exercise cannot make up for:
http://mashable.com/2011/05/09/sitting-down-infographic/
EDIT: If you want to live healthy it's really about changing the whole lifestyle, including dieting and the way you work(standing desk!). Just 4 hours a week won't cut it.
I've just read the Four-Hour Body by Timothy Ferriss. It seems on the face of it like ridiculously valuable material, if true - like what the completed version of Michael Vassar's proposed reboot of dietary science would look like at the finish point if dieting turned out to be more susceptible to Munchkinism than in my wildest dreams. Ferriss also talks the rationalist talk quite well in this book, much more so than in Four-Hour Workweek; he cites the experiments and occasionally says things like "I spent a lot of money on this and I expected it to work and it didn't work at all" or "I tried this and it seemed to work and I have no idea why it worked and I think it was probably a placebo effect."
Does the LessWrong hivemind have an opinion about 4HB? Has anyone tried it and found that it doesn't work, or that it does work, or that it works but not as well as Ferriss thinks it should work?