Something like 9 percent of students get above a 4.0 at graduation. I have no idea how many would get 4.25+ (which would round to 4.3), but it can't be more than one or two people every few years and is probably pretty heavily major dependent (some majors tend to give less A+s than others).
My daughter has a 4.47 (4.0 unweighted). She's took 2 honors classes as a frosh; 3 honors and an AP sophomore year and is taking 3 honors and 2 AP her junior year. Next year her plan is to take 2 AP and no honors.
I think the no of students who can get >4.0 depends on how many AP and honors options a school has. My daughter has taken many, but not all that she could. She hated Chem H, so wasn't about to take Chem AP. Doesn't love history, so no Mod. Eur Hist AP. OTOH, despite not loving history, she's enjoying her US Hist AP class. Since it's req'd for all students she felt the class discussions would be better in an AP secn.
Getting A+s at Caltech is extremely hard, even when compared to the already difficult task of getting an A in most classes. Getting mostly consistent A+s is basically impossible, not that it means that the occasional legendary Techer doesn't do it. There is some degree of luck involved. Sometimes a "generous" curve (or just everyone else doing badly) makes it so what might seem like a not very high grade becomes an A+ (often if you do better than everyone else in the class.) Sometimes a professor accidentally makes a final easier than he meant to and a bunch of people get perfect scores on it. Most professors will at least give you an A+ if you get > 98%. This is not always easy even if you know the material well enough to get close to perfect on the exams since sometimes the sets are held up to a pretty high standard of rigor.
Anyways, even med schools aren't expecting any A+s from Caltech, so it doesn't really matter.
I can't imagine being able to get A+ grades consistently here at Caltech. I know I've gone out of tests feeling that I did absolutely everything perfectly, and I did...except for that little minus sign...that cost me 5 points.
The frequency of A+'s definitely differs by major. In most of my math classes here at least 15% of the class got an A+, and there are a nontrivial number of math majors with 4.2+ GPAs in their major.
In other majors-ChemE comes to mind- this appears to be not at all the case.
"Harder" classes (in the sense that they're officially given more units by Caltech) are weighted proportionally for your GPA. This just means that classes like EE 52 and ACM 95 that are 12 units instead of the regular 9 units hurt your GPA even more than they would otherwise. Even though the average GPA in math is probably higher than the average GPA in mechanical engineering, mechanical engineering is likely a much easier major for everyone who isn't a math major.
^^depends on the majors. some (like math/cs) are totally doable (because you can use math courses to fulfill cs requirements and vice versa). it's also pretty straightforward if your second major is in the humanities.
but most double-majors are pretty impossible unless you want to take like 70 units a term