Q: 4 iphones using one apple id?

You have two choices that you should research well before making a decision.  The newer choice is to use Apple's "Family Sharing".  However, many of us with only a few family members find that the older, better option is to simply:

• SHARE a single AppleID account to login to the iTunes Store on ALL of your Mac and iOS devices (this will allow you all to fully share purchases with up to 5 family members)
• Each family member should use their OWN Apple ID for iCloud, iMessages, FaceTime, and everything else.

An important note is that when you setup a new iOS device, it will ask you for your AppleID during setup, and if provided, login/setup ALL services with that same AppleID.  This confuses the issue and makes most users not understand that the App Store, iCloud, iMessages, FaceTime, Mail/Contacts/Calendar are all completely separate services that can each be logged into using a different Apple ID (even on a single iOS device).

I definitely strongly recommend you each have your own Apple ID.

Here is how Family Sharing works:

Family Sharing - Apple Support

+1

To add to my own post, there is a very important reason to never use the same Apple ID for Mail, iCloud, iMessages, or FaceTime -- you will ALL see each other's data including iMessages!  That's right, if you use the same AppleID on all of your phones, you will typically receive each others iMessages.  Your kids will see your iMessages, and you will see theirs. 

Thanks.

Use the simplest way.

The person that has the purchasing Apple ID & password go first.

Setup the iPhone.

Now, each other family members create an unique Apple ID with his/her own valid e-mail address

Create new Apple ID through here: https://appleid.apple.com

Setup the iPhone.

After everyone has finished, except the one who has the purchasing Apple ID go to

Settings > iTunes & App Store > Apple ID: > Sign Out

then sign in with the purchasing Apple ID & password from above.