Has the download always been slow or did this just start out of the blue? Did you change anything around the time this started?
I assume you have tried bandwidth tests from a couple different places(speedtest.net, speedtest.googlefiber.net, DSLReports, etc) and all show the same result. Do you have any other devices on the network that you can test with? Assuming all device show the same result (which I would expect since the PS4 and iPhone already do) and since your ISP has run basic tests on your line, then I think the culprit is most likely the router (or modem, if you have one).
Router issues can be software or hardware. If it's hardware, then replacement is the only real option. Simple software faults can be corrected by restarting the router. The next step up is a factory reset (make sure to note any special settings, passwords, etc because they will be reset). If that doesn't work, I would test with another router ... maybe something that can be easily returned if the issue ends up something else. Was your router provided by your ISP? What is their replacement policy if a router is faulty?