Is there a recommended antivirus software for iMac running OS 10.13.4?

No anti-virus software is recommended by most knowledgable Mac users.

Read this article for details on why you don't need to download software and what you should be doing to protect yourself:

Effective defenses against malware and other threats

I would NOT recommend AVG.

None is recommended.

Mac can take care itself.

Download apps only from the developers' websites.

Hello! You should only download apps from developer websites, but if you are not there is an anti-virus called AVG. It is not the best but it does surprisingly well for an anti-virus that is free!

For some interesting reading on AVG, try Top 258 Reviews and Complaints about AVG Antivirus

AVG is also NOT recommended.

Have a read here: Antivirus protection and Mac OS X