Embedded Systems/Firmware Course Work
All of them.

My daily work requires elements from all four. I'd say for entry level positions you might get away with operating systems and computer networks, but employers are also interested in familiarity with different devices, and understanding the compilation process is also a big advantage when optimizing for size.

For a proper career, you want theory more than anything else -- every week a new framework is invented, and any hands-on experience gets worthless rather quickly.