Which is the only continent with land in all four hemispheres?

Africa, which is crossed by both the equator and the prime meridian - so it has territory north, south, east and west of the two lines that divide the globe. The crossing point of the two is in the Atlantic in the Gulf of Guinea, not on land, but the continent straddles both.

It is also drawn far too small on most maps. The Mercator projection, designed for navigation because it preserves angles, inflates areas towards the poles - so Greenland appears comparable to Africa when Africa is about fourteen times larger. The United States, China, India and most of Europe would fit inside it simultaneously with room to spare.

That distortion has had real effects on how the place is imagined, which is why alternative projections keep being proposed and why Boston's public schools switched to Gall-Peters in 2017 to some ridicule. Every flat map of a sphere is wrong in some way and the honest question is which errors you can live with - Mercator's are useful at sea and misleading in a classroom.