I'm a registered developer and have ran into that issue last night.
By any chance are you running any software firewalls?
Could be wrong on this, but it appears the iTunes authenticates with Apple for valid "developer status" (via I believe it was Verisign?). My Kasperksy Internet Security caught the outbound net traffic (to a Verisign server) shortly after I initiated the beta4 update. I blocked it without thinking, then after quite a bit of wait received a 3014 error. Repeated again w. the same result. Thought about it. Temporarily allowed my Kaspersky to allow all out bound traffics and beta4 went through without any hitch.
Good lucky mate!