4 Month tours? No. Wouldn't work for the Army. I'm not knocking the RAF - I am fully aware of some of the guys/girls in KAF/BSN working 18+ hour days for 4 months at a time, but there would be problems for the Army doing that.
Each time an AO is handed over, a massive amount of information/intelligence is lost, and has to be regained by numerous familiarisation patrols, taking up time/effort of the new troops in. 4 Month tours would simply exaggerate the problem. The Afghans already get tired of having their UK partners switching twice a year - where is the incentive for them to build relationships if each year they will have 3 different units working for them.
I believe that longer tours are coming; indeed in some of the larger HQ's 9-12 month tours are the norm.