The fourth season of Mad Men had its UK premiere last night on BBC4/HD at 10pm, but could only muster 350,000 viewers. This is slightly down on the third season premiere of 370,000. The show has never been a ratings juggernaut on either side of the pond, and the slip for BBC4 was relatively negligible. It perhaps didn't help that (a) Mad Men's been brought forward by over half a year to air in the more competitive autumn, (b) I haven't seen much promotion for Mad Men this year, and (c) it was up against a Derren Brown special on Channel 4.
I would guess that the missing 20,000 were instead lured to watch the Hero At 30,000 Feet event and will catch Mad Men when it's repeated. Incidentally, my review of Mad Men's premiere will be up tomorrow at Obsessed With Film.