Actually, the degree can be 3 years or 4 years or however long it took you. Visa requirements just say college degree or equivalent.

And, even if you don't have a college degree, you can still get a work visa as long as you can prove 3 or more years of related work experience.

Other ways to teach in Japan involve other types of visas.

Dependent visa
Cultural activities visa
Student visa
Spousal visa
Working holiday visa

The first 3 need special permission from immigration and are good for only part-time work.
The other 2 allow FT work.
The last one is for certain nationalities only.