This post is not relevant to previous posts but since I am reading members posted about game size and Pro's 4K above, I wanted to give an insights.
Game size
IIRC GTS CBT game size is about 20GB. I believe that size is stripped version from the retail version. Because you know the primary reason PD release this version: only to test their dedicated servers performance (Sport Mode) but all inputs from players are welcome, too. So I would expect ~30GB-50GB max with retail version -- the Blu-ray (dual layer) capacity. This does not include day 1 update, but please note since it is an 'update', an existing file maybe replaced, and not existing ones will be created.
Example: If day 1 update size is 5GB and 1.00 disc version is 40GB, total game size would be 43GB. Where is the missing 2GB? It replaces the files from the old version.
Or just prepare the worst, 70GB.
@torque99 estimation is also makes sense. Expect when GTS released so many new contents that would be touching 80GB. CoD IW on PS4 already 90GB in my HDD (all DLCs installed).
If you guys an active gamer I suggest you use 1TB internal HDD or purchase new external USB 3.0 HDD at 2TB or more. Please note the PS4 system WANTS a dedicated hard drive, not partition. It cannot be used for Windows PC unless you format the HDD. You also cannot partition them.
Game size with 4K
I am aware about the Forza 7 issue but somewhat relieved when knowing that the OG Xbox One is not going to download all 4K assets. I am not sure about GTS, but I personally believe the PS4, even Pro or not, will download the same game data.
4K
For starters, the 4K term used by Sony for the PS4 Pro is actually to promote a higher visual fidelity. Not *always* a native 2160p resolution as we know it. But all those
power users perception about 4K by default is a resolution. Also, power users do not believe PS4 would able to output a native 4K resolution because they simply knows PS4 OG/Pro uses a *weak* CPU. This is where all misleading begins.
Why *always*? Because the hardware supports *native* 4K output. But what about the games?
Games that supports native 4K output on PS4 Pro:
http://www.neogaf.com/forum/showthread.php?t=1289840
If not native, they would use a rendering technique called checkerboard rendering.
Please note that checkerboard rendering IS NOT an upscale like you imagine a TV would stretch or scaling a larger Image Size in Photoshop. It is a different rendering technique. First time I am knowing this technique (Killzone Shadow Fall, Rainbow Six Siege) I am really surprised because on layman terms you can produce higher fidelity of image with cheaper cost than going full native 4K. It has trade-offs but this technique is golden for developers.
Someone already writing a nice and great explanation about CBR:
http://www.neogaf.com/forum/showthread.php?t=1330505
Games that supports CBR (and upscaled to display output if it does not touch 2160p):
http://www.neogaf.com/forum/showthread.php?t=1324251
TL
R; checkerboard is good.
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Hdz even with the Pro patch, it doesnt change a lot, it quite suprise me, i thought it has more
Good read:
https://blog.us.playstation.com/2016/11/01/how-ps4-pro-enhances-horizon-zero-dawn/
Try play HZD with PS4 Pro with 4K, HDR-enabled output setting with 60" TV that has 10-bit panel. Compare with a tiny <32" full HD TV and PS4 Slim/OG. I was like AAAAARRRGGGHHHH.
If you already have television I just mentioned you can enjoy the red of Ferrari color on GTS lol.