Well, you have two things to consider: What is the Internet speed you have from Comcast, and what is the speed of your modem? For most streaming - and especially 4K streaming - you'll need 20mbps speed from Comcast. Minimum; bare minimum. Then, your modem has to deliver that speed to your devices (either via wifi or directly wired). Most internet plans and modems these days should be up to that task, but if you have older equipment, you may have problems.
Your friend is possibly right that your modem is older and doesn't support the speeds adequate for for reliable streaming. A good rule of thumb - depending on your budget - is to get a modem that delivers greater speed than your service. Over time, the speed you get from Comcast may increase, but if your modem can't handle it then it won't do you any good. Rather than continually replacing your modem, get one that is "future-proof" for a few years.