Incis0r is the man for getting the AF 4-year.
I didn't get on the HPSP train until the 4-year cycle had passed, but I did apply for and receive the AF 3-year scholarship.
Like the above, 3 years are more plentiful, so therefore easier to get. When I applied it was about 1:2 chance of getting the scholarship. I was a really competitive applicant, with a 3.5 and 21AA, but more importantly I interviewed really well and scored great on the leadership and EC components of the application (GPA and DAT are worth a good chunk of points, but aren't all of it).
Apply, and apply early if you're thinking about it. If you don't get the 4-year then you can basically roll over to the 3-year application. This coming year there are even 2-year scholarships available, but those aren't every year and still incur a 3 year payback period. The process overall takes forever so be patient and stay on your recruiter. Mine was amazing, but I've heard that's not super common.
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