Favourable vulnerable, first seat and the auction is uncontested.
1♦ 2♦
3♦ 3♥
3♠ 4♦
4♠ 5♦*
?
1♦ Precision - unbalanced two or three suited no five card major (1NT range is 12-15)
2♦ inverted
3♦ confirming diamonds but did not limit the range (implies clubs since no major has been bid)
3♥ ambiguous - advanced cue or no trump try
3♠ there seemed to be no clear agreement either a no trump try (without spades) or showing spade values
4♦ claimed by north to be RKCB
4♠ showing one or four (but obviously south has two)
5♦ to play and was made slowly, this was agreed although north claimed that other bids were also made slowly.
What are the logical alternatives for south?
South claimed that she knew that 4♦ was RKCB but misbid. She said that she was thinking of normal key-card (4NT) in which her response would be 5♠ and so mistakenly bid 4♠ thinking she was showing two.
To me this seemed an unusual argument as south does not have the trump queen and her response to a 4NT key-card would be 5♥ not 5♠.
I also thought it was significant that north, the bidder, rather than south explained the 4♦ bid as key-card. It seems possible given that north bid 4♠ that she did not interpret 4♦ as key-card.
Would you allow a raise to 6♦?