Q: What to do if the status on the ScholarOne system is "Under review" for over 4 months?

Peer review time varies by journal and field and can take anywhere between a few weeks to several months. however, generally speaking, it is always advisable to send an inquiry to the journal if status is "Under review" for more than 2 months. The reason for this is that  most jorunals have a high volume of submissions coming in and if there is no inquiry from the author's side, the editor might not really follow up with the reviewers procatively. This might delay the evaluation process for your paper. 

You should ideally have sent an inquiry to teh jorunal long ago. Please write to the journal immediately and ask for a status update. Keep following up every 2-3 weeks till you see a status change.

Related reading:

• How do I write an inquiry to the editor about my manuscript's current status?
• How to write an email to the editor inquiring about the current status of my paper?
• 6 effective tips to help you communicate better with journals