4GB header? Who would let an anonymous person on the internet send you an email with a 4GB header.
Figure out how to do it with reasonable limits and then I might worry.
You mean, do it within the 'reasonable limits' which aren't there in the default package or in any versions such as provided by eg Debian...?
My point is, the right answer isn't to say "that will never happen" and get surprised down the road, the right answer is to say "that shouldn't happen" and do something about it.
So, sure...don't wait till the header gets to 3.9GB. Absurd. But 100MB? 10? 1? I think I'd get antsy at 1MB. Especially since RFC2822 implies lines can be no longer than 998 characters. So...you know...someplace between 998B and 1MB would be reasonable to punt. Especially at the time the code was written.