I assume that if you are using a quadrature encoder (A and B phase outputs) that you are building some nature of servo positioning system.
Using a simple circuit such as what can be done with a CD4013 or CD4040 to divide the incoming frequency of both phases by 4 will not preserve the phase relationship of the original A and B signals. This will confuse the crap out of the quadrature decode mechanism in the PLC.
If you are using a quadrature encoder and "looking" at only a single phase, then I assume that you are only interested in counting shaft revolutions. If this is the case then, I'd ask, "why are you using an expensive encoder when a cheaper one would be fine?".
If you are only interested in counting shaft revolutions, and cannot use a more appropriate encoder, then a simple divide by 4 circuit might be a reasonable method to better match your system to the PLCs capabilities.
Gary d