In an "even parity" system, a 0 is stored in the parity bit if there is an even number of bits in the byte; if an odd number, a 1 is stored to make the total number of bits even. In "odd parity ...

So 4 is an even number. An odd number cannot be shared equally into two groups. For example, these 3 cubes cannot be shared into two equal groups. You have a group of 2 cubes and a group with 1 cube.