Well, the A330/A340 were developed as the one programme - they are essentially 2 and 4 engined derivatives of the same aircraft.
I do believe that ETOPS restrictions were a lot tighter when the A340 was being developed, but with these being relaxed, there are now many more routes open to 2 holers that only 3 and 4 holers could previously operate.
There is also the 'safety net' of having more engines to rely on should things go wrong, and less problems due to assymmetrical thrust. This has to be balanced by increased maintenance costs with having more engines. Indeed, 2 engine ETOPS aircraft have greater engine reliability records than 4 engine aircraft.
Still, many airlines, like Virgin, Cathay, and Lufthansa are happily flying the A340, and the -500 and -600 series are a definite improvement over the hairdryer powered -200s and -300s.
I don't think Airbus have suffered too much in sales, as the A340 has been a good seller, and the A330 a very good seller.