Which is better for a bright living room?

The C4. Brightness Booster and the Alpha 9 processor hold up better against daylight and window glare. The B4 is fine in a dim room but looks flatter in a sunlit one.

Is the C4 worth stepping up to from the B4?

Yes for gaming and bright rooms. You get four full HDMI 2.1 ports, G-Sync certification and a brighter panel. If you watch in the dark and use one console, the B4 covers the basics.

Which has better gaming features?

The C4. Both do 120 Hz with VRR, but the C4 adds Nvidia G-Sync certification, a 0.1 ms response time and four full-bandwidth HDMI 2.1 inputs.

Does the B4 come in a size the C4 does not?

Yes. The B4 comes in 48 inches, and the C4 in this listing is 65 inches. For a small room or desk setup, the 48-inch B4 is the practical pick.

Do both support Dolby Vision and Dolby Atmos?

Yes. Both are OLED sets with Dolby Vision and Dolby Atmos, and both run webOS 24 with the same streaming apps.

Neither has a clear edge on panel life. The C4 gets five years of webOS updates through LG's Re:New program, which keeps its software current longer.

LG OLED evo C4 65-Inch TV vs LG B4 OLED 4K Smart TV

The LG C4 is the better TV of the two. Its Alpha 9 Gen7 processor, Brightness Booster and four full-bandwidth HDMI 2.1 ports give it a brighter, cleaner picture and more room for consoles and soundbars. The B4 uses a weaker processor and a dimmer panel, but it runs the same webOS, supports Dolby Vision and 120 Hz gaming, and comes in a 48-inch size the C4 does not. Buy the C4 for a bright living room or a serious game setup; buy the B4 for a dark room or a smaller space where the step up buys little.

Choose the LG C4 if you want the brightest picture LG's OLED line offers at this size, four HDMI 2.1 ports for a console plus a soundbar, and the longer webOS update commitment.
Choose the LG B4 if you watch mostly in a dim room, want a 48-inch OLED, or would rather put the difference toward a soundbar.

Prices

LG OLED evo C4 65-Inch TV: $1,238.91 at Best Buy (checked today)