Posted 2011-November-03, 21:15
I misunderstood Bradley's question, I thought he was just soliciting opinions about the best use for this 4♣ bid. I've investigated further, and GIB doesn't have an explicit meaning for the sequence, so it's just falling into the default "undefined bids are natural" rule. I'm discussing with Georgi whether there's a missing rule for this sequence in the bidding database.
This also prompted me to look at some other NT sequences: 4♣ after a suit response to Stayman also falls into that default rule, and 4♣ after a Jacoby transfer is strong and natural; I think both of these should be splinters. And it plays that 4NT after a Jacoby transfer is Blackwood for the transfer suit (e.g. 1NT-2♦-2♥-4NT is Blackwood for diamonds), which is wrong in several ways.
I'll bring these up with him as well.