From what I hear, there weren't a lot of copies of IV and V manufactured. Which is a shame, especially for V - it doesn't rely on NES bosses at all, they're all completely new. It had Super Game Boy support as well, if I remember rightly. What's disappointed me, though, is that Capcom hasn't decided to use the GBA's extra screen width to give us more screen space. One reason the GB Mega Man games seem to be so difficult is because of their limited screen size.