Are you wired or wifi? If the latter then I would guess that is the reason why. It took me a good ten minutes to get my PS4 connected wifi to a router 6 feet away. If I try to connect upstairs the signal isn't even strong enough for PS4 to find it.
My PS3 has no problem with above 90% upstairs so I am pretty sure it is a PS4 issue and not your internet.
If you are wired then I'm afraid I have no idea.
If you cannot run cables to hard wire maybe think of investing in powerline home networking.
NB please note I am not stating any of this as fact. As such I don't feel I have to supply proof or that I am a liar. lol
Thank you
Cheers Happy hunting.