For it to have been the 4th ''greatest'' game of all time as an old 70's game (Yes, you're old xD), the keyword is that it must have been genre-defining, kind of how the Super Mario series have been, they pretty much defined the sidescrolling genre as we know it today. The Super Mario games weren't the most polished nor did they have the best graphics or entertainment value (when I was a kid, we could play Super Mario Bros 3 on our NES all day for a while, then get bored with it and not play for a month, but I bet I didn't even play it 1/10th of the time I've spent playing WoW.)
The Super Mario games also appealed to children and adults all over the planet, and I'll bet anything that the reason I've never even heard of The Oregon Trail until now is that I'm Scandinavian. It sounds VERY ''American only'' to me.
#4? N***a please.
It should not be up there.
PS. I was born in 1988.