Instead of the endless search for the perfect static posture, just keep moving the various joints now and then. Adjust your elbow or wrist or the desk or the chair every now and then. This can make you look "unprofessionally" seated if you take it far enough so hopefully your workplace doesn't mind that.

Even if you are moving between multiple so-called "bad" postures, you'll be much better off compared to just being static in a "good" posture for 4 hours.