Now South has no good bid. 4♠ shows 3 for some reason, 5♣ shows four of them (!) and bidding a red suit had one of those 6+ suit, rebiddable suit, 4 card suit explanations and made GIB support it with its singleton.
a) Why can't GIB just bid 2/1 with its GF 6-5 hand?
b) Why does 4♣ show such modest lengths in the description? Presumably GIB wouldn't really remove 3NT to show a four-card suit. Which brings us to:
c) Why on earth would 5♣ require four of them? What is opener's rebid here that says "leave me alone, we're in a misfit auction"? 4NT was blackwood, of course.