Which continent extends to all four hemispheres?

Well, I wouldn't say it "extends". It's just that both boundary lines ... the one that separates
the northern and southern hemispheres and the one that separates the eastern and western
hemispheres ... happen to both pass through Africa, putting parts of that continent in all four hemispheres.

Sure, Africa is a good-size land-mass, but its size is not the reason that it crosses all of the
lines into all of the hemispheres. Asia is much bigger than Africa, but is almost completely
in the northern and eastern hemispheres, with only a tiny part in the southern one.