A PSP accessory advertisement in the latest issue of British trade mag MCV contains an odd selling-point: that it's compatible with the PSP-4000. A console which does not exist.
See for yourself in these pics. It goes a little beyond "typo", since they actually created a little bubble logo for the console. So it's either an elaborate, embarrassing mistake (which, to be honest, seems more likely), or accessory manufacturers 4gamers figured that nobody outside the "industry" would bother reading MCV's mag, so they could slip mention of a fourth iteration of the original PSP in there and hope nobody noticed.
Whichever it is, you can see the original ad below.