Is it normal for a Path Tracing render to take 50x longer at 4k than at 1080p resolution?

Hi, I’m rendering a single still image using Path Tracing lighting through the Movie Render Queue with the settings below. When I render at the default 1920 x 1080 resolution, the resulting image looks great and the render takes just over 5 minutes to complete, which is awesome. But, with the exact same settings except the resolution is at 4k, it literally takes over 4 hours to complete. Is this normal? What am I missing and why is there an insane increase in time? I expect about a 4x increase in render time but not 50x. Any suggestions? Thank you!