What they meant of course was not whether they could be "at the job(site)" for more than 4 hours a day, but that you could only really do actual (productive) work for 4 hours a day. The rest, so my friend claimed, is almost always filler. My friend also claimed that they believed this was true almost regardless of the type of work you did. Even people doing physical labor don't actually "work hard" for much more than 4 hours - you need breaks (lots of little ones, or maybe a few long ones).
I still don't know if they're right about this. Personally, I've always preferred the maxim about "work long, hard or smart: pick 2". Either way, I know that across society, not just in IT related fields, we do not honor these ideas about work in any meaningful way.