This looks like a specific textbook answer, so your teacher is probably looking for those specific four factors. Since I don't have your textbook, here are the four variables I would use to classify the different oceans:
1) Latitude - it's location from North to South on the globe determines much about the ocean, including temperature, the location of currents, and the type of species that can live in it.
2) Depth - Take a relatively shallow ocean like the Gulf of Mexico, and a deep ocean like parts of the Atlantic and Pacific (with depths up to 30,000 feet) and they are vastly different oceans in terms of the kinds of life they can support
3) Salinity - the salt content of that ocean, or lack of it, is a prime determinant of the kinds of life that is sustainable there
4) Temperature - closely related to depth and latitude, but the shallow warmer seas, tropical ones, that is, support a different kind of life than the colder deeper oceans.
Hope that helps!