Particular aspect that i could compare in the field of religious study of two different types of Buddhism?

You could compare liberation from the point of view of the Hinayana (theravada) with that of the Mahayana. http://en.wikipedia.org/wiki/Buddhism#Theravada

I don't possibly see how you could write a paper 4000 words long (that's about 12 pages long ... double-spaced)

Rebirth is the Buddhism word for many lifetimes. Reincarnation is NOT a Buddhist concept, except for the exception in Tibetan Buddhism where the word refers to the rebirth of a high master ... but there's not 4000 words to write about THAT difference.

Really, the differences between the various schools of Buddhism are very minor. Far easier to write about how they are similar.

And Buddhism isn't really about belief anyway. It's about what you practice, developing the skills of compassion and training the mind ... and seeing what the leads to. It's experiential, not a belief-system.

It would be far easier to talk about the differences between Buddhism and Hinduism.

Suggestion. Examine the concept of No-Self versus some form of Self found within Buddhism. Look into Tibetan and Zen Buddhist ideas. Also relates to subtle difference between Rebirth and Reincarnation.