buy a ps4pro 1tb with fw 4.70.  (or anything <= 5.07 they say)
keep it disconnected from the network.

download fw 5.05 from (for example):
http://drivers.softpedia.com/get/gaming-consoles/Sony/Sony-PlayStation-4-Firmware-5-05.shtml ( https://www.google.com/search?q=ps4+system+update+5.05+download )
download the update file. md5sum should be f86d4f9d2c049547bd61f942151ffb55
format a 8gb usb-thumbdrive to fat32
put the downloaded file PS4UPDATE.PUP in H:\PS4\UPDATE\ (as seen here https://www.playstation.com/en-us/support/system-updates/ps4/#update-computer )

https://www.google.com/search?q=start+ps4+in+safe+mode
plugin the gamepad with a usb-cable and the usb-thumbdrive.
find the power button. it's on the front, middle row, to the left. hold it down for some 7 seconds. first there's a beep immediately, then another beep after 7 secs.
now you're in the safe mode menu. navigate to the "update from usb" menu, and make sure it says 5.05.
click and wait.

https://retropie.org.uk/download/
use http://rufus.akeo.ie/ to write the image to sd-card
unzip https://github.com/Al-Azif/ps4-exploit-host/releases/download/v0.4.5/ps4-exploit-host-html-v0.4.5.zip to the root of the sd-card.
boot rpi2, press F4 when prompted. (for extra terminals, user/passwd is pi/raspberry)

for temporary networked testing:
sudo ip -4 addr add 1.1.1.1/24 dev eth0
cd /boot/ps4-exploit-host
sudo python -m SimpleHTTPServer 80

the permanent fix for retropie:
edit /etc/network/interfaces
add at bottom:
auto eth0
iface eth0 inet static
        address 1.1.1.1
        netmask 255.255.255.0
        gateway 1.1.1.33

edit /etc/rc.local
add lines before the last ("exit 0")
cd /boot/ps4-exploit-host
/usr/bin/python -m SimpleHTTPServer 80 &
cd -

connected a network cable directly from the rpi to the ps4.
the power usb-cable for the rpi can be plugged into the back of the ps4. that way it will power on/off with your ps4 automatically.
on the ps4, set the ip-address to 1.1.1.2/24 (255.255.255.0)
go to the web browser, surf to 1.1.1.1, make a bookmark for conviencence.
click on the webpage for the 5.05 firmware, then on the HEN option.

put a nice game .pkg in the root of a usb drive. exFAT formatted should work.
go to the ps4 menu system -> debug game -> install package, install it, exit, and play.