As others have said, a £2k bike is more than suitable but any more than that really does open up a whole world of satisfaction! Assuming that he wants an off the peg bike and isnt interested in custom builds (buying separates) and also assuming that he wants comfort over out and out performance, I'd point him in the direction of the Cervelo R3 which is a superb bike, i have a previous model the RS and its just sublime. Designed to be comfy over long distances without sacrificing too much in terms of aerodymanics, the higher head tube does pitch the body slightly more upright than other models but thats becoming increasingly common these days as sportives etc grow and grow in popularity. I love it, it encourages fast cornering, has very big burly chainstays and spaghetti thin seat stays, a comfortable ride that helps transfer all the power to the back wheel.
This model [url=http://www.jejamescycles.co.uk/cervelo-r3-ultegra-road-bike-id82031.html?gclid=CIO9n6G0obsCFXMgtAodsCEAAw] is from JE James and has good spec for £3k. Cervelo predominatly sell framesets only so the frames really are some of the best on the market, you can gauge the quality of the frame by comparing the cost of the frameset, and then the built up bikes, to the other brands. Wheels will always be the weakest point on an off the peg bike but if he is insistent on spending the full £4K, a pair of Reynolds Assualts or something similar will keep him happy.
If he is ok with buying unseen, the likes of Canyon and Rose offer superb value for money but I dont know too much about their higher end models other than the [url=http://www.canyon.com/_en/roadbikes/bike.html?b=3293] Canyon Ultimate CF SLX has been ridden on pro tour stage races by the likes of Katusha and now Movistar so has got to be pretty good!. A friend has one and loves it for all the reasons I love the RS.
Also worth a look, BMC GranFondo, Trek Madone and Giant TCR ranges are all very good bikes and will have something to suit most budgets.