Is it possible to replace the 3.1V6 engine in a 95 Beretta with a 3.4 V6 without replacing the transmission if they both use a 4 speed automatic transmission?

Yes it is a direct fit. Although it is best to refer to the engines as 3100 and 3400, respectively, so as not to confuse them with 3.1L MPFI (1990-1994) and 3.4L. They are all different engines.