I agree with dundurn. The poster doesn't share a great grandfather with the person he is talking about. The two great grandparents were first cousins and as it has gone down three generations on each side, the poster and the person he is talking about would be 4th cousins. Also agree, if they were brothers, the relationship would be 3rd cousins.
Sorry Goudsoulette but you and whiskeysheri are 100 percent wrong! Read the first line of the web site he offers up - if people SHARE a common ancestor...
Now read the post jd_here and the person he mentions have DIFFERENT great grandparents who are first cousins. Now YOU count the generations down.
If you're still not convinced - imagine that you are the great grandparent and one of your first cousins is also a great grandparent. Your children and his would be second cousins to each other, your grandchildren and his would be third cousins to each other and your great grandchildren and his would be fourth cousins to each other.