Developers add value in so many more ways that heads down concentrating on code. A senior engineer should spend the least amount of time coding compared to everything else they do during the day. If they don't, do you really need a senior engineer? How much of a 4 hour coding session is something you needed specific expertise in and how much is mindlessly typing the syntax with a big of logic peppered in?
If your tasks can all be done truly asynchronously without blocking someone else, they probably aren't very exciting or special. Having worked for many years in both office 9-5s and global async teams, async teams are incredibly frustrating to organize if the work requires any collaboration. In a perfect world sure your 4pm is my 8am and we can work there, but factor in real life, people leaving early, coming in late, and it becomes incredibly annoying to deal with in practice. It takes much more management overhead, and you truly have to ask if the benefit you get from having engineers async is really worth that extra overhead. I would argue in most markets it just isn't. I would take a 20% worse engineering staff all working together than the alternative of a better staff split into different time zones. This is coming from someone who lives in Iceland and has had to work async for the bulk of my projects in the last few years. I found the lack of human contact and collaboration so boring that I took a big paycut to work with people again every day.