"Yeah, I got a 4" -- can he do that?

[SpeakEasy]

This is more a rules question, but also a really interesting situation.

$1000 max buy-in, $5/10 blinds. SB and BB are heads-up on the river. Each has around $800. Pot is about $400. Board is 8-2-2-4-2.

SB bets $200. BB goes all in. SB asks, "you got a 4?"

BB says "Yeah, I got a 4." SB thinks a while. BB says, "Here I'll show you," and turns the 4[img]/images/graemlins/spade.gif[/img] face-up on the table. BB then taps the other card (still face down) and says, "Can't see this. This one is special." He then turns the 4 back face down.

Question: is it legal for BB to expose one of his cards while the hand is live? Does this kill his hand?