As 52Mag92 told you, a 4th gen test is considered conclusive after just four weeks in some places, I'd like to add that, this forum is US based and we must advice according to CDC's guidelines which state that any HIV test is conclusive only if taken 12 weeks after an exposure.
But you can take them before that, 3rd gen will give you an excellent indicator about your status around 8 - 10 weeks after exposure and 4th gen 4 - 6 weeks after exposure. A result given in those times is very unlike to change, but only a doctor, based on your risk level and his experience could advice you to skip the confirmatory test after 12 weeks post exposure.
Dr Handsfield left something wroth reading about the subject.
http://www.medhelp.org/posts/HIV---Prevention/what-is-the-window-period-for-hiv/show/1704700