Using the technique of
singular fibers of C∞ stable maps, we give a new proof to the theorem, originally due
to Rohlin, that the oriented cobordism group of 4-dimensional manifolds is infinite
cyclic and is generated by the cobordism class of the complex projective plane. A
byproduct is a new and transparent proof of the signature formula, originally due to
T. Yamamoto and the author, for 4-dimensional manifolds in terms of singular
fibers.