Sorry, I can't provide you with dates. (I suspect that the BBC4 schedules have yet to be finalised).
However I can confirm that Only Connect is due to return at some point during the year, as Series 5 is listed on the website of the company which makes the programme, Presentable:
http://www.presentable.co.uk/television.html
That page states that you'll get 20 episodes in the series which, according to Wikipedia, is more than in any previous series:
http://en.wikipedia.org/wiki/Only_Connect
Chris