It will be likely be how they doing with the Vita and PSP. You can have only two portable console attach to one account (1 Vita 1 PSP, 1 Vita 1 PSPgo, 2 Vita....)

So I guess it will only be 1 PS3 and 1 PS4 per account.

You have to create a user with your PSN Account in both consoles... that's how that works with PS3 today... limited to two console (no more than that).

But we don't know if that will change with PS4.

Yes, as long as your PS+ account is on both machines.

that's how it works with the PS3, but we don't know for sure that it'll work the same way with PS4

While some have pointed to the capability to use one account to activate games on 2 PS3s this feature isn't restricted to PS+, it's the way most of the digital content works.  It used to be that you could activate on up to 5 different PS3.  It was essentially what MS is calling Family Sharing except tied to consoles rather than accounts (and that it wasn't meant to be shared with others).

Since Online isn't a feature of PS+ currently I reckon the strongest indicator would be the automatic downloads feature - it's freely assignable to any console that you have your PS+ account on but can only be active on one PS3 at any time.  I can have mine active on either the PS3 in the bedroom or the lounge but not both.

So my guess would be that this is the level of flexibility that PS+ will have on PS4 - play online multiplayer from any PS4 you are logged into but if you want to online mp from 2 PS4s at the same time you will need 2 PS+ accounts.

However, as some have already said I think the only really honest answer is, "we don't know yet"

@Tuscaniman99
Just because it's common sense to you does NOT mean it's true, so stop giving such inaccurate information.
Source or it did not happen.