What should I do if my paper has been under review for more than 120 days?

Peer review is generally the most time-taking part of the journal decision making process, and it usually takes anywhere between 2-4 months on an average. In fact, in certain fields, it can take even longer. However, I would recommend that you send an email to the editor asking by when you can expect the peer review to be completed. That way, the editor might also send a reminder to the reviewers and the process can perhaps be expedited.

You can find a template on how to write an inquiry to the journal editor in this article:

How do I write an inquiry to the editor about my manuscript's current status?

You might also like to read the following post: 

What should I do if the review process is delayed and the editor is unresponsive?