On covering and quasi-unsplit families of rational curves
  We study extremality properties of covering families of rational curves on projective varieties. Among others, we show that on a normal and Q-factorial projective variety of dimension at most 4, every covering and quasi-unsplit family of rational curves generates a geometric extremal ray of the Mori cone. 
L. Bonavero, C. Casagrande, and S. Druel