Ok ok ok... so here's the deal:
After a GI, you get 27 or 28f of advantage, where your opponent cannot block. (I'll test it properly next time I play)
Tira's 4B+K is i26, meaning after a GI, your opponent cannot block this move, so long as you do it straight away.
The only way your opponent can counter your attack is by using a re-GI.
The active window for a GI is not that big, so if your opponent throws out a re-GI instinctively, there's a good chance that the re-GI will be too early and your 4B+K will hit anyway.
Your attack will only fail if your opponent either leaves their re-GI very late, or actually reacts to and recognises your attack, timing the re-GI properly to counter the move.
All this makes 4B+K a very good attack to use in JS after a GI, and also because 4B+K B 236K BE does about 70 damage and gives you a chance to transition into GS. The only drawback is that this will cost you a full bar of meter.