Is it possible to have two accounts from different regions on a PS4?

I used to have two accounts on my PS3, one in the UK and another in the USA.

I used each one of them to download Exclusive Content which was unavailable in each region. Sometimes I would buy games from USA, sometimes from UK. In PS3, it wasn't a problem. I can play both of them without any problem. Is this possible in PS4 too?

So let's say I activate two of my accounts in my PS4 and set both of them as primary account (I think it's possible). Can I play all of my games in UK account? For example if I buy a hard copy of Assassin's Creed Black Flag from USA, can I play it with my UK account?

Yep, it's all the same as on PS3. Primary account shares all content with other accounts and you can have multiple primary accounts. Excerpt:

If a user activates a system with his or her account, other users on the system can use applications and enjoy some PlayStation Plus benefits from that account, even if that user is not logged in.

If multiple users have activated the system as their primary PS4 system, deactivate it for each user.