Ivy leagues don't really have specific prerequisite courses, they just want to see that you challenged yourself with a rigorous academic curriculum in high school. So four A-levels is more impressive than three, but it isn't necessary. Something to keep in mind is that Ivy leagues emphasize well-roundedness far more than schools like Oxford and Cambridge (who don't really care about extra-curriculars, and just want to see you excel in the academic course you're applying to)
There's the SAT reasoning, which tests english and math and has an optional essay (which is required by nearly every Ivy) and then there's the SAT subject tests, which tests subject specific knowledge. The subject tests are not required by all schools; there are there for you to demonstrate proficiency in a certain subjects. You can also choose to take the ACT instead of the SAT. It's the same idea and content, but there are some differences in the structure of the test, so choose the one that fits your skill set.
I think A-levelsare more in-depth than AP courses. Ivy applicants will probably take upwards of ten AP courses throughout their four years of high school, while UK students have GCSEs, then spend two years intensively studying three or four A-levels. I think both GSCEs and A-levels will be considered by the schools, since the US high school system is something like mashup of the two.