Do you have any devices that are connected 'wired', if so are you able to do some speed tests on those to see if you are getting the speed you are expecting. You could then compare that with a speed test from the PS4 to see if that is getting a much slower speed.
If you don't have any wired devices then try doing a speed test from a wireless device when it is right next to the router, and compare that with what you get on the PS4.
There are a few things you could try to see if it is Wi-Fi interference causing the problems.
Split the SSID's on the router, this will allow you to give the 2.4GHz and 5GHz networks different names, so your Wi-Fi devices will then see those two networks separately, you can then try connecting to one or the other to see if one gives a more reliable connection than the other.
Change the channels that your router broadcasts the Wi-Fi signal on, you can use free smartphone Apps like this to see what channels are being used within range of your smartphone, you can then change your router to use channels that are less congested.