Bahrain 2010 will feature 4 WC competing in the same GP, if Kimi had secured a drive, this would be 5. This is fairly rare, the record seems to be in South Africa 1968, where 5 champions did line up: Clark, G. Hill, Brabham, Surtees and Hulme. Sadly this was to be Clark's final GP.
At the other extreme, 1959 featured no champions competing, as Hawthorn chose to retire.