There's also the DS being released at the end of 2004, which was less then 4 years after the GBA was released. Yeah it still had a lot of games that got released in 2005, and Nintendo called the DS just a third pillar when it was launch, but for all purposes, the DS was meant with the clear intention at Nintendo to replace the GBA and be the system to stop Sony from stealing their market with the PSP.
So the Wii U wouldn't even be the first Nintendo system to last 4 years before its successor was released and the GBA was doing much better as well.
Oh and this is more or less a confirmation about the two platforms, one OS thing, isn't it?
Pretty much, it fits Iwata's comments and Nintendo merging their handheld and home console hardware teams a few years ago very well. It the best way for Nintendo to continue supporting 2 systems since it's allows easier development for games that receive handheld and home console version and give them the option to even outright release a game on both platforms at once.