Roger is spot on. There are several problems to overcome before dreaming to perform carving jibes:
1) I would try to use a 40 cm fin with a 6.x m2 sail on your Futura 131 to learn carving jibes. You might have problems when keeping the board at speed and banked on her rail during a carving jibe when you have a too big a fin (48 cm) with a too small a sail. Especially in choppy water...
2) You would need at least a 6.5 m2 sail to start and sustain planing and it should be perfect for your 75 kg in 15-20 knots of wind to learn carving jibe.
3) I'm afraid that you can't do that with a 4.2 or with a 5.2 m2 sail in 15-20 knots of wind because you won't be able to initiate the carving jibe at full planing speed. You would need 25+ knots wind with a 5.2 m2 sail or 35+ knots wind with your 4.2 m2 sail. If you cannot build enough planing speed at the curve's entry, you simply cannot do carving jibes.