Naturally, getting a very high GPA in your fourth year will be crucial to a successful application, considering the results of you last three years. If I were to look from an admissions committee's point of view, I would see a candidate with significant clinical experience, but also one who has had a downward trend in marks following a solid first year. Therefore, aceing fourth year will help show the admissions committee that you are fully capable of handling a difficult workload.
With that said, having two consecutive years of not-so-great marks is going to hurt in any case. If the schools you apply to perform a computer screen on all incoming applications, (ie. only consider interviewing the applicants with the top 50% GPA) then your GPA might be lowered to such an extent that you would fall below the threshold. In this case, you won't receive an interview, and having clinical experience counts for nothing.
If the committee grants you an interview, it would be because your GPA is sufficiently high. You'll then have a chance to explain those two slippery years to your interviewers, who may bring it up to the other members of the committee. But first, you've got to get those fourth year marks way up. Having a strong MCAT (ie. greater than 30), will also prove that you can stand the academic rigors of med school.
During your interview, you'll surely be asked why you have decided to switch from nursing to medicine. I think there may be an unspoken bias against professional students applying into medicine. The rationale is firstly that you have inadvertantly taken a spot away from someone who truly wanted to be a nurse, and secondly that it shows that personal commitment and job loyalty may not be very high.
I want to emphasize that this viewpoint and bias is not fair, but I believe it may exist. It may not exist at the schools you are interested in, but because of that possibility, you should be absolutely clear in your interviews why you feel nursing is not what you want in a career, and why you believe that medicine is the right choice. If it's what you really want, I personally would love to one day work side by side with you.
As an aside, my classmates include former occupational therapists, physiotherapists, and pharmacists. However, I don't think there are any former nurses.