Ps4 – borrow and play a friends ps4 game on the console

Yes, local accounts are those that exist - are/have logged in - on the same console, whether created there or elsewhere.

The Playstation Plus subscription is tied to the SEN account, but that account can specify a single PS4 console as its primary PS4, which enables other (non-PS+) accounts to access everything on the console.

So any account using that console (the one specified as the primary by at least 1 PS+ subsriber) can enjoy the benefits of PS+, when using that console. That includes multiplayer.

I assume though that multiplayer is not available to local accounts if you are online and playing multiplayer on someone else's console. That part isn't exactly clear. But your (non-PS+)family members / housemates could definitely play multiplayer on your console with only your PS+ subscription when you're not using it.

This FAQ on playstation.com is fairly detailed:

PlayStation 4

You can activate one PS4™ on your Sony Entertainment Network account (this is known as your Primary PS4) and temporarily activate a second PS4 on your account.

PlayStation Plus benefits available to other non-subscribing users on your Primary PS4:

• Online multiplayer (subject to any parental control settings on sub-accounts)
• Play downloaded games purchased with PlayStation Plus discount
• Play downloaded IGC games

PlayStation Plus benefits not available to other users on your Primary PS4:

• Purchasing PlayStation Plus discounted products from PlayStation Store
• Online game save storage
• Auto patch download
• PlayStation Plus exclusive early access to game trials

PlayStation Plus benefits available to you on a non-subscribing friend’s PS4:

• All of your PlayStation Plus membership benefits are available to you when you use a friend’s PS4.

PlayStation Plus benefits available to your non-subscribing friend when you use their PS4:

• Play downloaded games purchased with PlayStation Plus discount
• Play downloaded IGC games

PlayStation Plus benefits not available to your non-subscribing friend when you use their PS4:

• Online multiplayer (subject to any parental control settings on sub accounts)
• Purchasing PlayStation Plus discounted products from PlayStation Store
• Online game save storage
• Auto patch download
• PlayStation Plus exclusive early access to game trials

When you sign into your Sony Entertainment Network on a friend’s PS4 the system is temporarily activated on your account. Please ensure that you correctly sign-out of “PSN” on your friend’s system to keep your account secure.

If your “PSN” account is inactive on your friend’s PS4, the system will be deactivated automatically from your account and your friend will lose access to any of your downloaded content. This content can be deleted from their system hard drive by going to Image (Settings) > System Storage Management > Applications. Next, highlight the game to delete and press Options > Image (X button).

Source: http://faq.en.playstation.com/app/answers/detail/a_id/5030

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:

1. 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.

2. Sign-in to your SEN account.

3. 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.