Q: Do you need a student visa to study in Japan?
A: Yes. You will need to obtain a student visa in order to study in Japan. If you already have a visa, changing it is just a matter of filling out the appropriate paperwork and submitting it to the immigration office.

More information:
Ministry of Foreign Affairs of Japan’s website

Q: Does the 4-years at HAL “count” as a degree to get a work visa?
A: I believe so. According to the Ministry of Foreign Affairs of Japan’s website the educational requirement to have a work visa in an Engineering field is that:

The applicant must have graduated from or completed a college or acquired equivalent education majoring in the subject relevant to the skills and/or knowledge necessary for performing the job concerned, or the applicant must have at least 10 years’ experience (including the period of time spent studying the relevant skills and knowledge at college, “Kotosenmongakko”, senior high school, the latter half of unified secondary school, or during specialized course of study at an advanced vocational school (“Senshugakko”)).

Since HAL is an accredited technical school I believe that it meets that requirement. For a definitive answer, however, I would recommend contacting your local embassy.

More information:
Ministry of Foreign Affairs of Japan: Appendix 1