Asking the Editor-in-Chief about the reason of the review delay

I have submitted a manuscript to one of the top-tier journal of my field at six month ago. The recent status of the paper is "under review" which status is there for last four months.

My question is: In that situation is it ok to ask the Editor-in -Chief of that journal about the reason of the review delay?

Also Could anyone please help me by giving a useful reference to a template of such mail (to the Editor-in-Chief), which ask the above question to the editor in a polite manner.