Four hours per day of work where you are in high focus is about normal. You will see that professionals across many creative fields only work 4 hours per day.
There are a lot of posts here from developers talking about how they are more productive than their peers working less hours. Really, they probably have the same capacity for hours, but the less productive developers are carrying a lot of baggage in attempting to manage themselves (trying to push themselves when they shouldn't be.)
I think you could actually put in more. The trick is to observe your own natural rhythm. For example, energy and your ability to focus is like a wave through the day. For most, I think the time of the most energy is early in the morning and then it declines from there. You aren't burning hours so much as you're burning that fuel in your brain. But if you put in your 4 hours early, then you could probably take a good break and get another good 90 minute session. You could also find other tasks that are much less cognitive demanding to fill your day. If you put in your 4 hours that you believe are more productive than what your peers put in, then fill out the rest of the day with things that are lighter and less "forced."
If you are running a business, then after your creative work you still might have email, quotes, meetings, marketing, billing and a long list of other things to do. It could be pretty easy for a business manager to knock out 4 hours of creative work per day and then still fill out the rest of a normal working day with other tasks.
If you didn't have these other sorts of tasks, then maybe you could work out a side project with your employer. After you do the "forced" work, then maybe there is something that you could work on which is more a "scratch an itch" type of project. This could be something that you see as a glaring problem for the business which also happens to be something that you are highly interested in. Maybe it's something that's a different area of expertise that you might want to move into in the future.
Or maybe your 4 hours is all you can do without burning out.