Netflix requires 25 Mbps per 4K stream , but plan for 50 Mbps for one stable 4K stream and 100 Mbps for two 4K TVs plus normal household traffic. HD needs 5 Mbps; SD needs 1 Mbps.
You also need a Netflix Premium plan and a device that supports HEVC and HDCP 2.2. If 4K still stutters on a fast plan, the bottleneck is almost always the TV's WiFi adapter — use Ethernet.
These are per stream . A household with two 4K TVs needs at least 50 Mbps just for video, before phones, calls, or downloads.
Netflix uses adaptive bitrate streaming, it constantly samples your connection and downgrades to HD or SD if it sees congestion. With only 25 Mbps, any background traffic (phone backup, OS update) will trigger a downgrade. Aim for 50 Mbps to keep 4K stable.
Most "my Netflix isn't 4K" complaints are WiFi, not ISP. A cheap smart TV with a 2x2 802.11n adapter caps at 50-60 Mbps in real conditions and drops to 15 Mbps two rooms away. Plug the TV into Ethernet, even cheap Cat5e cable gives you a stable gigabit link.
You also need a Premium Netflix plan to unlock 4K, and your TV/streaming stick must support HEVC + HDCP 2.2. If everything is in place but bitrate stays low, run a speed test from the device itself , laptops over WiFi are usually the bottleneck, not the ISP.