Post your equipment details and signal levels if you like and someone will help you.
Also, hook your PS4 up to the router via ethernet. WIFI is not ideal for gaming.
The first thing you should do is hardwire a computer into your gateway and run a speed test.(make sure the speed test is using a local server) If your still not getting the speeds your paying for unplug your gateway for a minute and then plug it back in and wait for it to load and do another speed test. You do not specify if you have a comcast modem, do you have a gateway or a modem and wireless router seperatley. Do you have anything hardwired into your Modem? How Far away are you from the Modem? Keep in mind Streaming/Gaming will require more of your bandwidth then other devices. Depending on what everyone in your house hold is doing you may need faster speeds. Cant really help you unless you provide more info.
-Have you hardwired a computer to speed test?
-Disco anything hardwire to modem( I have seen Sono's,Apple routers, Dvd players hardwired causing speeds to drop but once unplugged speeds jump up)
-Is your PS4 connected wireless or hardwire?(If wireless are you close to gateway?)
-What kind of modem/Gateway? Rental or Customer own? is it Dualband?(on the front you would see 2.4GHZ and 5GHz or 2 wireless lights)
-If you have a Dual band sometimes they broadcast with the same name and it could cause issues with speeds or range so I would recommend separating networks.