Don’t say anything negative about your previous or current company, manager or co-workers. A negative response will make the hiring manager think you’ll speak badly about them too. Equally, don’t mention anything related to being under-paid, over-worked, under-appreciated, or give the impression that you’re job-hopping.
The best approach is to say something concise and positive. For a Project Manager Job a good response could be:
I really enjoyed working at my current company, for the last four-years I have delivered multiple projects from procurement to deployment, and I have developed many strong business relationships. I am looking for more management responsibility, but the opportunity for growth in this area is limited. I feel leaving my job is the next step to achieving my goal.