Just to clarify, will you be using Retropie to play retro games on the Pi 4? Which display monitor will you be using? It sounds like you’re building a portable project that is to be powered via LiPo battery/powerbank. You may want to take into account the power requirements for the display.
You can view the power supply requirements for all the Raspberry Pi models here: https://www.raspberrypi.org/documentation/hardware/raspberrypi/power/README.md
According to the link above, the Raspberry Pi 4 requires 600mA at Typical bare-board active current consumption and 3.0A for Recommended PSU current capacity.
You may also be interested in checking out the Game HAT which runs on a 18650 lithium battery: https://www.littlebird.com.au/products/game-hat-with-raspberry-pi-3b-and-retropi