digital phase shift

Is there a simple digital way to make a 90 degrees phase shift for a digital signal?

is it a simple signal (as in doesnt change frequency). If so just delay it for 1/4 of the timeperiod shoud do it

No, It has variable frequency.
Actually we are going to multiply a signsl's frequency by 4, and to do this we have two original signals named A,B which are a linear glass scale encoder's outputs and they have 90 degrees phase difference.
we xor them to make the frequency twice, and so I need to make another more xor to reach to 4 times multiplied frequency.