According to this site, you are able to do what you described for you and your friend to share games. However, in order to play your own games, you will be required to have an active connection to the internet at all times, which is less than ideal.
However, because your own PS4 will not be registered as your primary PS4, no one besides you can access your games on your PS4. However, everyone who has access to your console will be able to play your friend's games, since that would be the account with your PS4 listed as its primary. Likewise, anyone that plays on your friend's PS4 could access games that you have purchased on your account.
Savefiles are kept on a per-account basis, so any games that your brother plays on his account will not affect you or your friend's savefiles in any way.
I've answered this same question here: https://gaming.stackexchange.com/a/271692/51332
Short Answer is that as long as you don't have a "Primary" PS4 attached to your account, getting the second one in the future will not affect you. You can play on the current PS4 as a sub account and upgrade in the future without losing anything (like trophies data). On the other hand, he can attach his account on your PS4 to share his library with you. (You'll need to redownload the games of course).
Now in regards with just having one account to share, that's possible too. From the same page:
On how many SEN accounts can one device be activated?
You can activate a PlayStation system on as many SEN accounts as the highest number of possible local user accounts on the system.
This means that you can activate a PlayStation 4 or PlayStation 3 on up to 16 SEN accounts at once and you can activate a PS Vita, PSP or PS TV on one account at any one time.
There's a link on the page as well to explain how to share your account's content with your "friend's" console:
Create a new Local User account on the PlayStation 4 and go to [Settings] > [PlayStation Network] > [Sign in]. Note that system parental control settings may restrict this option.
Sign-in to your SEN account.
Select ‘No’ when you are asked if you want to activate this PlayStation system as your ‘Primary PS4’. This is to ensure you are able to continue to access your PlayStation Network content on your own PlayStation 4.
You and your friend now have temporary access to your PlayStation Network content from your friend’s system.
Using these steps, you can make the first account, tie it to the first PS4 and buy the majority of games onto it. Make the second account, tie it to the second PS4, log-in and save the first account's credentials to the second PS4 as well to share those games between the two systems.
Best Answer
You should be able to sign-in to the new PS4 with your son's existing PSN details (which should have been created on the Family PS4). Signing with the existing details will retain the trophies and other data, but you will have to setup any console-specific parental controls again (like Play time limits and age restrictions). You will also sign-in as the main (master) account holder in order to setup the parental controls.