You can multi-task with Multi window on Samsung Galaxy S4. It is a nice feature on Galaxy S4. However, not every application supports multi window ability. You can’t even select them in this case. But only for those supported natively. How to enable all apps on Galaxy S4 multi window? Samsung Multi Window Manager can help but with limitation. And root access is required.
It is not limitation of Samsung Multi Window Manager itself. But the application that used in multi window. For example, Samsung own Calculator application does not support multi window natively. By forcing it, you get non-working calculator due to layout issue. Therefore, better ask developers to support multi window in their applications natively. While waiting for the code changes, use Samsung Multi Window Manager then.
Download Samsung Multi Window Manager here.