Won't need to link!
Just make sure PSN account is the same as it is on the ps3 for ps4 so then you won't need to worry about the SE ID cause it is already or should be linked to PSN Account and should have SE account ID already entered!
There are already some details in SE's PS4 Beta Test Overview article. It sounds like people should be using the same Sony account for both PS3 and PS4.
As for the PS3 to PS4 transfer, I agree more clarification is needed about whether it will be offered after the one-time April 11 transfer. I can see people playing on a PS3 now and not buying a PS4 until after that transfer date.
But to repeat and be clear, the transfer service is only for if you have only a PS3 registration before, and only want a PS4 registration after. (The capability to play on PS3 goes away with the transfer.) If you want both options for login, SE says to buy a PS4 copy of the game and add its registration to your existing service account.