What a bottleneck is
A bottleneck is the part that sets the limit on performance. In games it is usually the graphics card, and that is fine. It becomes a problem when the processor cannot prepare frames as fast as the graphics card can draw them, because then part of what you paid for in the graphics card goes unused.
Processor limits show up most at 1080p and in fast competitive games, where frame rates are high. At 4K the graphics card does far more work per frame, so the processor matters less.