In Japan the numbers 4 and 9 are considered unlucky. The way the word for '4' is pronounced is apparently similar to the way the word for 'death' sounds. Meanwhile, the word 9 also sounds like a word that describes a form of suffering.
Certainly for the number 4, this has been notably skipped in Japanese camera model designations, although it's a bit inconsistent. Canon skipped from Powershot G3 to G5 models, omitting the 'G4', for example. But Canon was quite happy to produced the EOS 40D, 400D and 450D models. Olympus moved its flagship DSLR from E-3 to E-5, skipping 'E-4'. Then again Olympus used the number 4 in its E-400, E-410, E-420 and E450 DSLR camera model numbers. Fujifilm went from its S3 Pro DSLR straight to the S5 Pro - there was no 'S4 Pro'.
In fact I'm struggling to think of an important Japanese camera model name that is based around the number '4'. Can you think of any?
Oh, and if you add the two unlucky Japanese numbers of 4 and 9 together, what do you get?
:)