Q: What can I do if my paper has been "Under review" for 4 months?

You haven't mentioned the statuses before your paper moved to "Under review." Assuming this is external peer review, it's still quite a long time. You should definitely send a polite email to the journal inquiring about the current status of the paper and by when the reviews are expected to come in. As a rule of thumb, always send inquiries to the journal if there is no status change for more than a month. 

Here's a template that you can use to write to the journal:

Dear Dr./ Mr./Ms. [Editor's Name],

I have submitted my manuscript titled [insert manuscript title and id] to your journal on [insert date]. The status has remained "under review" since [insert date when this status appeared.].  Since it has been over four months, I was wondering if there is any specific reason for the delay. It would be great if you could let me know by when I can expect the next status change.

Sincerely,

[Your Name]

Do read the following articles as you might find them useful:

6 Effective tips to help you communicate better with journals

Additionally, we have  A practical handbook of templates for communicating with the journal which is really useful to write to the journal under different circumstances. Do take a look at it.