If you stream actual 4K content on your TV, it will at times use 41Mbps at 30 frames per second. If you watch 4K 60 frames per second on YouTube, you will need more than 60Mbps.
I have gigabit internet and there is no question that 4K picture quality improved over my previous (much lower) service. However, I have a 77 inch LG OLED TV, which really does highlight any flaws.