Well the datasheets for these kind of counters should explain modulus capablity, but basically you have the option with internal reset circuitry and access by device pins to have the counter "reset" at any count up to the 16 count capablity of a 4 bit counter. Example, Modulus 10 would have the counter with states 0-9 and then reset to zero, making it a decimal counter.
The word modulus(Lat. nominative singular) and the modulo(Lat. ablative) both refer to the base of a number system. The number of unique states that a counter may have before the sequence repeats itself is the modulus of the counter. We call it a modulo-n counter if it has n-states. You may find the following link helpful: https://en.wikipedia.org/wiki/Modulo
naseeam, did it occur to you to look up the word modulus?
4-bit modulus means 4 bits of information 0-15 count
A counter means counts up from 00 to it's limit, in this case 1-16 counts.
See the attachment whether you can get an idea from it
Well Q1, Q2, Q3, Q4 are the four outputs (4 bit).
When you give a clock or a signal to the 4 bit counter IC it starts count it by binary.
When you feed the 1st clock it shows up the Q1 ON.
When you feed the 2nd clock it shows up the Q2 ON.
When you feed the 5th clock it shows up the Q1 & Q3 ON. (5 = binary ‘0101’)
So it can counts up to 0 to 15 bcuz it is 4 bit.
For a decimal system this 4 bit is more than enough to show the digits because decimal system uses 0-9.