Although I'd have ideally wished 'Indy 4' had been simply called 'Indiana Jones and the Kingdom of Skulls' (or even the still slightly-alternative 'Indiana Jones and the Kingdom of the Skull'), I had a think about what I'd have liked if it had more closely reflected the feel of the 'Indiana Jones'-less 'Raiders of the lost Ark' title instead -

If this had been the case, I think I would have wanted it to be another 5 word title, to sound similar to the 'Raiders' one. So TMBTM's 'Kingdom of the Crystal Skull' fits the bill better, compared to the look and sound of say, either 'Kingdom of the Skull' or 'The Kingdom of the Skulls'. (Even though it still reveals the 'crystal' aspect)