I think it may be 1. 1st number is 4, plus 2 gives 6 on the end. 2nd is 5 minus 1 gives the 4 (next to the 6), 3rd is 2 plus 2 gives other 4 so 2 minus 1 =1.
So +2, minus 1 from first to last working towards the middle.
Hope this makes sense. It may not be right, but it works