Mega Man 4 was released in December 1991 in Japan and in January 1992 in America. Some days ago, Protodude has found an old scan with Electronic Gaming Monthly’s coverage of CES 1991, in which we can see a beta version of Mega Man 4 with a removed Boss (Sphinx midboss?), some different details in the levels and an early Stage Select! You can read more in the original post.
Thanks a lot to Protodude for the contribution!