Darn I assumed a N-S and E-W hemispheres. But now realise there are an infinite number.
So. The first, second and third darts all have a 100% probability of hitting the same hemisphere.
The fourth dart will have a probability that is in between 50 and 100%. If the triangle forming ABC is 90 degrees then the probability will be 50%; and if it forms a 180 degree straight line, then the probability is 100%. So a sliding scale between 50 and 100 depending on where dart C lands and the angle formed by ABC.
