Note: These instructions put the PS4 (or whatever is at that IP) onto a DMZ that has direct connection to the internet. There's no filtering or protection, it bypasses Shield services. 

Set PS4 to Static IP Address: 

1) Set the PS4 to a static IP outside of DHCP range (Click here to learn how to set the static IP Address) 
     Note: The below IPs will be different if you have changed the default IP address of the Shield
      
      Router Mode
      Static IP: 10.10.10.50
      Subnet Mask: 255.255.255.0
      Default Gateway: 10.10.10.10

      Bridge Mode
      Static IP: X.X.X.50 (X=Your Routers Network)
      Subnet Mask: Your Routers Subnet
      Default Gateway: Routers IP Address

Apply Custom Firewall Rules:

1) Login to Shield Web GUI
2) Network -> Firewall -> Custom Rules
3) Copy and paste the below two rules into the bottom of the file
      iptables -I FORWARD -s 10.10.10.50 -j ACCEPT
      iptables -I FORWARD -d 10.10.10.50 -j ACCEPT
4) Click "Submit"