It's not about "having post processing", it's about specific effects that literally hide the graphics of the game behind so much blur that you're basically only seeing what's directly in front of you clearly.
and about comparisons with Forza... uhm, have you read the thread title? I compare it to Forza 4 because I think that the game looks literally worse than an Xbox 360 racing game that runs at 2x the framerate and with expensive 4x MSAA (MSAA is almost like partial super sampling, it renders the edges of geometry at 4x the resolution in this case)
the game looks like absolute ass as soon as all the effects that hide that are gone. you can't compare that to the effects used in Mario Galaxy, these work no matter how you move the camera or how fast you go.
here another screenshot that someone threw my way
look at the low res textures, the basically non-existent texture filtering, the low detail meshes of objects, barely any grass mostly a flat green texture, low res trees, absolutely awful anti aliasing...
this game looks like if you took a PS3 game, ran it at 1080p and slapped the worst post processing AA on there that they could find.
it literally only looks decent as soon as you drive at high speeds so that the motion blur hides this or if the whole screen is covered in raindrops
Forza Motorsport on Xbox 360, which runs at 60fps and uses 4x MSAA (yes I'm repeating myself but it's worth remembering) looks better in many ways than this 30fps PS4 game, of course it also looks worse in some regard, the resolution is lower for example... It is also worth remembering that the physics in FM4 run at around 300hz, which is very CPU intensive, and while I have no direct info on this game, I would bet it runs the physics calculations at nothing more than 60hz since the driving physics are extremely arcadey (which isn't a bad thing, but just to keep that also in perspective here)
now imagine if Forza Motorsport 4 didn't need to run at 60fps, and didn't use such an expensive form of anti aliasing... it already only looks slightly worse than DriveClub in clear weather.
and all of this wouldn't be an issue, if it wasn't for someone making a whole thread about how this game still looks better than even Forza Horizon 4 on PC at max settings and 4K... which simply is bullshit of the highest degree, and people who argue that in here either use photo mode screenshots which use higher settings than the in-game graphics or they post gifs where the whole screen is filled with rain