Can I download and play games from a friend's PSN account on a PS4?

Sharing your digital games, apps, and other downloads with your family and friends is possible with the Primary and Non-Primary Account [...]

In this example we are using Ethan and Katie who both have their own PSN account IDs. Ethan also has a bunch of digital games which he bought on the PSN Store using his PSN account. Objective: He wants to share these digital games with Katie, so this is what he does:

Both Ethan and Katie can now play the same game at the same time on PS4 (A) and PS4 (B) with no restrictions.

Benefits of activating a PS4 as Primary:

• Anyone who uses your Primary PS4™ system can enjoy applications you purchase from the PlayStation®Store. Other users on the system can also enjoy some PlayStation Plus benefits from that account, even if that user is not logged in.

Limitations:

• This method of sharing only applies to digital games and apps. The sharing of disc games will still require you to physically lend your discs to others.
• When you log into a non-primary PS4 with your PSN ID, you will need an active Internet connection so that games and apps from your PSN account can be authenticated by PSN.
• You can activate only one PS4 system as primary per PSN account.
• You can deactivate a PS4 system only from that system.

The whole explanation and the picture belongs to gripitaustralia.

There is an official article about activating and deactivating you Primary PS4 on playstation.com.