I'd second what Klinger said. You will get a different story from the dealer when you are buying a high mileage bike to the one that you will get when you are selling one. In truth, 4000 miles per year is trivial for any big bike let alone a BMW. But the price guide dealers use knocks money off for mileage over (I believe 2k per year) and this becomes a self fulfilling prophecy when it comes to part ex's..
So the question you have to ask yourself is whether you are going to keep the bike for a long time and do high mileage, or whether its going to be a weekend toy. If the latter, then get a low mileage bike. If the former, then it doesnt really matter either way.
But 20k on a year old bike with full serice history should be no problem in terms of wear. I would be inclined to carefully check the physical condition (ie frame, wheels etc) since that sort of mileage implies some sort of commercial use. Check for drops, accident repairs etc.