I'll give the official statement:
"When its ready"
Now, to expand on that. We already know that there is going to be a b5. There are quiet a few bugs that popped up and are squashed in b5. Will there be a b6? That depends on many many bugs pop up in b5. B4 has been out for a while, and there are already a lot of things that have been marked as fixed in the b5 release. Since they haven't released b5 yet, I believe that they are really trying to make b5 the last beta. But, again, it all depends on how many bugs pop up in b5.
