I don't know who to choose!?

This is a really hard choice. You have good reasons to invite all four. You care about your friends and don't want anyones felings to be hurt- and you want to invite people who are sociable and fun. What about finding a way to earn the money for the extra 2 people? Maybe if someone in your family is planning to buy you a present (grandparent or aunt), you can explain the situation and ask them to help pay for the party?