I believe the number "4" is bad and usually skipped in Japanese culture. Not sure of the validity of that statement, I just think I read that in a few places.The same argument was used to say that Canon wouldn't release a 1D Mk4...
Makes sense, Lumix went from the LX3 to the LX5, and the gf3 to the gf5.
So we should probably take the 4 off the table.
Also, like others have said, a lower number indicates higher end model.
The aversion isn't so strong that the number can't be used at all, but it is really bad, and I agree that they would avoid making the bulk of the name of a product the number 4, for fear of losing potential customers due to superstition. The worst part about it is that it has a multiplicative effect with other superstitions. To illustrate how bad it can be, in most rule sets for Japanese Mahjong, if all four players discard the same wind tile on the same turn, the game is instantly declared a draw, originally implemented for fear that if the game were to continue, all participants would die
. Sadly, this isn't even that rare statistically; assuming that a player was willing to discard the fourth wind at all, it would occur approximately once every 750 games if all tiles had an equal chance of discard. In reality, once strategy is factored in, the true rate is much higher.
In the case of the 1D Mark IV, there are two things going on. One is that the number 4 is auxiliary to the name, and two is that "Mark IV" is canonically said in transliterated English even in Japan, made very easy to market as such because there's no direct Japanese translation for the word "Mark" used as such, so it no longer sounds anything like "death".