Awful Falafel
Banned
It's not confirmed: see that radiotimes interview with the creators above. They'd like to do more but nothing is certain, and they say it's definitely possible this was the last series. Also Martin Freeman has been heavily implying he's done with the part in recent interviews. He doesn't want to play the part for too long and felt this was a nice point to end it on.
That article you linked to is based on a misquote. Mark Gatiss has since explained that they planned out the Christmas special and series 4 at that point, not series 4 and 5. People often repeat that article but it's simply based on a misunderstanding.
Whelp, I'm full of shit. Please ignore me. Though man, a retraction or update in those articles would be nice.