I'm thinking of getting with a girl that has had a crush on me for 4 years
Are you going to ask her out now?
I am not sure if I feel the same way about her.
There is nothing wrong with not reciprocating feelings for someone if you do not feel that way. It would be bad to ask her out if you are not going to be committed to her fully.
I'll keep her around in case I change my mind.
Doing that is worse than turning her down. You should not lead someone on like that, getting her hopes up for something that may not happen ever. How would you feel if someone did that to you?