.4, just be glad you gave up gatwick. I'd say most aircraft are 145/150 IAS at 4dme there.
2 this morning (A330's) slowed to 160kts without being told to. First one I told him of his error then instructed him to maintain the 160 to 4 dme, next time I check his speed he's 140 at 5!
Heathrow had a loss of separation last week with 2 aircraft on base leg caused entirely by the distraction of taking corrective action with an aircraft that slowed early.
You can't even follow a pattern of aircraft types or airlines that are particularly bad, in my experience it comes down to individual crews not following their clearance. Filling in reports isn't the way forward, with our new electronic reporting system it takes me half an hour to file a report, that's my fatigue break. The best way is to reposition the offending aircraft or send it around but it's not always possible when you've traffic close behind.
If 160 to 4 cannot be achieved then we should be talking about bringing in a new standard and accept the inevitably lower movement rates but it's dangerous to be issued and readback a clearance of 160kts to 4 when you have no intention of following it.