As far ask I know, with some special exceptions, there's no distinction between courses taken in the summer and courses taken over the fall/winter term. It wouldn't hurt to do what jhan said and check though. However, it might take a while since you're capped to six units per summer term (there are two), and even if you can snag an exemption for those one-term, 4+ unit courses, I don't think you'll be to take more than two in a term.
Not to mention the very limited selection of courses available to take in the summer. Unless you plan on taking a lot of electives, it looks like there'll be trouble your way. Since a full September-April year consists of thirty units, you'll need two and a half summers to make up for one school year of courses, so if you exclusively take summer courses, you'd take 7.5 years to graduate.
Of course, this is reduced a lot since you only said you wouldn't be able to take a full course load over the year. If you even take half a course load, that'll reduce your time a lot. I just did some rough math in my head, and I think you could finish in four years if you took fifteen units (half a full course load) over the year.