Let (M, g) be a Riemannian 4-manifold. The twistor space Z → M is a CP 1bundle whose total space Z admits a natural metric g. The aim of this article is to study properties of complex structures on (Z, g) which are compatible with the CP 1 -fibration and the metric g. The results obtained enable us to translate some metric properties on M in terms of complex properties on its twistor space Z.