Ps4 – Can one PlayStation Plus subscription be shared between multiple PSN accounts on the same machine

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

Acutally you can; my brother is doing it with a friend, but it's tricky. It's like buying plus, having your account on both devices and interchanging devices with someone else.

Take: Skynet (my brother's PS4), and Cerebro (his friend's); Charlie (my brother's account) and David (his friend's).

There's an option in the PS4 that says something like: "Activate Device" for "Game" and for "Video", to activate a main account on the device.

David has Plus privileges on his account but his 'Active Device' is Skynet, and vice- versa Charlie has Cerebro as his main device. And they both have their accounts on both devices (main and secondary). So on Cerebro are both accounts, Charlie and David. Charlie is the main account of that device but it's on David's house; and on Skynet too are both accounts, David is the main account, and it's in Charlie's house.

Basically David+ is playing with his account on Charlie's device, in his own house (as if he were a guest in his own device), and Charlie is playing in his house, with his account on David+'s, device.

 Cerebro (in David+'s house):     Skynet (in Charlie's house):
>     -Charlie (Main)                 -David+ (Main)    
>     -David+ (2nd)                   -Charlie (2nd)

This way David+ can play online on Cerebro and on Skynet, because his account has plus, and Charlie can only play online on skynet because the main account of the device has plus and enables all accounts of the device to enjoy the plus privilege of online play.

And yes: anyone of them can buy a game and download/play them on both devices.