Does anyone know when Season 4 starts? I think it is sometime in Autumn 2011 but I'm not sure.

At this time there is no confirmed date for the start of series 4, but 8th October appears to be floating around as a rumour.

sometime between October 5th-17th

IT USUALLY STARTS WHEN DOCTOR WHO FINISHES AND IT TAKES OVER THE TIME SLOT

There aren't confirmed an airing date for the brand new season of Merlin, but there are written on BBC's own official Merlin homepage (link), that it will air some time this October... ^w^
But there's some rumours about Merlin will air 8th October after Doctor Who has ended... ;)

it's been offically confirmed that episode 1, the revival of the witch Part 1; will air on the first of October 2011.

The first week of October!!here it is
link