Unfortunately a lot of the entry level routers don't have a very powerful cpu in them so they can't keep up with 100Mbps when applying all the firewall rules, routing tables etc to the traffic.
Definitely worthwhile getting a decent gigabit router if you're going to be doing 4k streaming and if at all possible, keep it all wired, don't do 4k streaming over wifi.
Something interesting I came across the other day when I upgraded my fibre to 100Mbps. I'm using an old PC for my router and it had a 100Mbps network card on the WAN side. I was getting approx 95Mbps up and down on the Speedtest. I changed the network card on the WAN side to a gigabit card and was then getting approx 105Mbps up and down, so I think the fibre links are a bit faster than what they actually advertise.