There's really only two possible reasons for buffering during playback. One is that the Roku is defective (unlikely but not impossible) and the other is a networking issue. Note that the network issue might not be your home network, but could be some problem your ISP is having. When you do an Network connection test, what speed is it reporting?
The 4800 has been (for me) pretty accurate in reporting it's network speed, up to the limits of either the wired or wireless access point used. For example, my home Internet speed varies between 600-700 Mbps. My 4800 reports 93 Mbps wired (which is accurate for a Fast Ethernet connection) and around 205 Mbps wireless (likely the limits of the hardware Roku uses for the WiFi radio).
Dan
Roku Community Streaming Expert
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