As I pointed out, I'm not convinced that mIRC should be making this decision. I'm not exactly totally sure of the details, but IIRC there were some issues raised a while ago about the possibility of adapters having both ipv4 and ipv6 support at the same time. In this case, binding to an address might bind to an adapter that COULD support an IPv6 address. Again, I'm not sure if this is possible, but this is one possible scenario. It just seems to me that, if you're enforcing a specific adapter/bind ip, you should enforce the protocol too.
In any event, the issue here is not whether mIRC should guess what protocol to use, it's the fact that mIRC can't resolve an ipv4 host when in IPv6-only mode.
