IT WILL NOT WORK BELOW 3.18
This method uses the OpenCMA/PS3 trick to allow you to use a PS Vita with a lower-than-current firmware(3.00 and above) as a remote play device for your PS4 eg. if you need a lower firmware for a game exploit.
1) Install OpenCMA from here http://wololo.net/downloads/index.php/download/1252 , try connecting to OpenCMA with your computers internet turned off to make sure it installed properly.
2) Connect your Vita with a lower than current firmware to your computer by usb cable.
3) Open Contant Manager on your Vita and wait for the transfer type screen, then unplug the Vita from your computer WITHOUT closing Content Manager, we need that to stay open, just press the PS button and swipe to the home screen.
4) On the PS4 go to Settings then "Playstation Vita System Connection Settings" then "Add Device" and it will give you a code.
5) On the PS Vita open the PS4 Link app and choose Remote Play and it will ask you to sign in, sign in and it will ask for your sharing preferences(I just chose no-one) then it will ask for the code you got from the PS4, enter it and you're ready to go.