You're considered a FT student when you have 60% or more of your faculty's normal full course load. 6 courses is 18 units i believe. So that's 60% of a full course load for faculties with 30 units. However, that's not enough for a few others (like science and engineering, I think?)
4 courses in the summer is considered full-time but students don't get the HSR pass in the summer anyways regardless.