Setting aside issues already discussed re the relevance of "positions" and the numbering thereof, the answer to your first question is yes -- C is the second note in the Bb scale, so assuming 4th position is built around the 2-note, 4th position on a Bb accordion would be key of C. Keep in mind that playing in C on a Bb box, you don't have access to either the 3rd note (E) or the 7th note (B) in the C scale.
As to your second question, builders or historians may be able to provide a "correct" answer. Practically speaking (and assuming by "top button" you mean the 1 button), it seems to me at least that continuing the "pattern" down to the 1 button doesn't provide much benefit (since not a lot of melody is going to be played at such a low pitch), whereas breaking the pattern provides some nice capabilities that wouldn't otherwise be available. Specifically (and speaking in terms of a C accordion):
1. The G note on the 1 pull allows you to end songs in the key of G (of which there are many) with a nice full G chord (1-2-3 pull) that actually contains a G note.
2. I've heard at least one player (Chris Miller) doing a traditional bass shuffle in G that relies on that low G note (1-3 pull / 2-4 push / 1-4 pull / 2-4 push, etc.). It's a cool sound that couldn't really be done without the low G on the pull.
Nathan - Yeah, anyone trained on pretty much any instrument except Cajun accordion thinks of "high" and "low" in terms of pitch. Despite that fact (and despite the conventional numbering of buttons on a Cajun accordion), the majority of Cajun accordionists I've played with tend to speak of "high" and "low" (or "top" and "bottom") buttons in terms of altitude / distance from the ground rather than pitch. Hence, I assumed Greazy's reference to the top button meant the 1 rather than the 10 (also because, as you point out, the 1 pull is the only real deviation from the "pattern"). Very confusing, but what can you do?