Anything multiplied by 10, 100 or 1000 is even and adding an even number to an odd/even number results in an odd/even number. Therefore only the last digit matters and every number will be odd except for those ending in 6, of which there are 3! = 6.
Ok mooman, I have got your point. Now: 7, 3, 5, 6. There are 3 odd numbers, that means that just 3 of four numbers can be used to get a four-digit odd number; so: 3 out of four numbers.
3 possibilities will be multiplied for the rest of available digits: 3*3*2*1 = odd numbers.
1 possibility will be multiplied in order to get the total of even numbers: 1*3*2*1 = even numbers.
Thanks.