It really just depends on what you want to teach. Many
principals like for you to be able to teach all four
subjects, but in my school the preference is on one subject.
Most, if not all, of our 8th grade teachers are actually
secondary certified in their subject areas.
Also, check out how the "highly qualified" stuff plays a part.
It IS possible to be certified, but not highly qualified.
You will need to satisfy both requirements.
A pain. I know.