I have searched for quite some time now and I cannot figure out how to input a year like 2000 and divide it by 4. Then test to see if it is an integer or a float number.

The modulo operator in C is %. It gives the remainder in a division between integers (doesn't work with floats at all)
For example 7 % 3 == 1. The quotient is 2 and the remainder is 1

This pseudocode is copypasted from wikipedia

if year modulo 400 is 0 then
is_leap_year
else if year modulo 100 is 0 then
not_leap_year
else if year modulo 4 is 0 then
is_leap_year
else
not_leap_year