I'm trying to do that thing where it increases the download speed but in order to do this it has to be a manual/custom wifi. What ip address do I use? My router's, my computer's, my ps4's, what? I've never done anything like this before so I have no idea where to go from here.
If you are not familiar with manual address settings, use the PS4's address, gateway, DNS and mask. Just make sure that the PS4 is switched on BEFORE you boot the router or you could get an address conflict.
Manual addressing won't make any difference to speed. It doesn't work that way. Do what Harry said and use an Ethernet connection.
No serious gamer would use WiFi. It can be too erratic.
You do NOT use the routers or computers IP address! That would really confuse everything. You use whichever IP address that you like that can talk to your router. The one that has been allocated to your PS4 by your router + 1 would be a good place to start. Do a PING for the address from your PC to make sure it is not in use already. If it is, add another 1 to it.
Manually setting your PS4 a static IP address will NOT increase the speed at which you can download. Your speed is limited by the fact that you are using wifi.
If you want to see a significant speed boost, run a network cable between your router and your PS4 and switch wifi off. Your download speed on your PS4 should increase by 50% or more.
Type 192.168.1.1 or 192.168.0.1 and it should open router address page