Yes, this sucks and you are justified in questioning why. And I think you should ask your parents about it. But be cautious. You don't want to accuse them of anything, instead you would like them to explain.
“Mom, dad, when I asked you for a PS4 you made me earn it. I was able to achieve my goal and that feels good. But when sister asked for a PS4, you bought her one. Would you please explain so I understand your thinking?”
There are a few possibilities:
They forgot they made you earn one (Yes it happens)
They made an arrangement with your sister that you don't know about. (a reward for a goal, a loan)
Double standard might apply
When your parents discuss it with you avoid reacting if you get upset. Parents aren't always right but they may resist if you point it out by yelling. And in my experience you won't be able to make them change what they do. You goal in asking is to clarify and understand so that you can put your mind to rest on the matter.
Now, if it turns out your parents expect more from you than your sister, even though it's unfair, you are better off in the long run. As much as it sucks your parents are teaching you a valuable lesson in finances. Your sister is not getting the same lesson and that fact will bite her and your parents in the butt in the long run. Your sister is just learning to be dependent. You are learning to stand on your own two feet. Whatever unfairness there is in not supporting you the same way they do your sister, is more than made up for by your independence.