Ultimately, at the market level pay rates are tied to productive value. We have a theoretical assumption of 40h/week, but almost everyone understands that it's fictional (and more people are learning that). Most people who are going home and getting the same work done in 2-3 hours aren't working less -- they're working more efficiently, or doing fewer other things (e.g. chatting at the water cooler).

For many types of thought work, 3-4 hours is pushing it anyway; the default assumption of 8 hours doesn't make sense with the heterogeneity of what different types of work actually entail.