game works fine, but when I enter a loading screen everything is blurry for a while, and then back to normal.

problem is, even if i shut down fallout 4, and enter rest mode on ps4, and then restart it, the blurrynes is there again for 10 secs, after that everything ok.

is this dangerous for my beloved ps4 or just some glitch that I dont need to be affraid of.

I do not have a PS4. It might just be the loading of the new content and at first it might just have a very low quality image as a place holder until everything is updated, possibly used to help maintain the capped 30 FPS.

might be but that doesnt explain why it does it after I have had my ps4 in sleep mode and start it again

Rest mode doesn't do anything such as clearing cache or releasing memory as the system is still technically running. You would need to do a full shut down and restart to test that. I couldn't sprint and the world wasn't fully loading for me the other night, luckily mine was fixed by just a reboot of the game. You may need to do more and do a full shut down which I have had to do with some issues I have run into. Also, if you just go into rest mode and the game is running with a problem the game continues to be suspended with whatever issue it had.

hmm could be, though I am not if I was clear enough, the second blurryness wasnt in game though, but at start where I log in to my ps4, that is the part I dont understand hehe.

In that case it's definitely something with your PS4 or tv. Try doing a shut down (not rest mode) good luck.