First of all, keep in mind that a call for an interview is NOT a job offer: all it is is a call for an interview. So as excited as you are, try not to jump the gun about that
Yes, you need to tell Med Center about your current position. Will your having 4 months experience look bad? Honestly, I really doubt it. If anything, they'd probably be happy that you have some RN experience under your belt.
As far as how it looks (possibly) leaving a job after 4 months...that, I don't know. But I do know that if Med Center truly wants to hire you they will make you an offer regardless of where you are working now or how long you are working there.
Ultimately you need to do whatever is best for you and your career. I would make two suggestions, which you are free to consider or ignore at your discretion.
First, if you do get a job offer from Med Center, leave your current job on the best terms possible--this means giving the appropriate notice and not burning any bridges. Nursing is a much smaller world than you realize, and you never know if you will cross paths with someone who used to work at the first job, or if you end up having to apply to work there again.
Second, I would stay at the Med Center job for at least a year or two. To job hop once early in your career is forgivable. To job hop twice in less than a year does no favors to your reputation.
Best of luck with your interview!