I use the list randomizer on random.org, and each number will get put into that list the number of times the ticket was purchased. Then I hit "randomize" and it will generate a list of those numbers in random order. The first four unique numbers to come up will win 1st, 2nd, 3rd, and 4th prize respectively. Any numbers that repeat in the first 4 will be ignored and I'll move to the next unique number to pick that winner. If you have 100 tickets, you have 100 chances of your number appearing in one of these slots. If you have 1 ticket, you have one chance.
Hope that helps. =)