Is it yet feasible to use a USB powerbank to power the Raspberry Pi 4B? Are there any that you've successfully tried and could recommend? Can a 2.4A USB-C bank work?
The Raspberry Pi 4 spec recommends a 3 A supply (15 W), with a minimum current of 2.5 A if downstream USB peripherals consume less than 500 mA in total.
Furthermore the MagPi benchmarks a power draw under load of 7.6 W "running a CPU-intensive benchmark while an HDMI display and a USB keyboard and mouse are connected" (note that they do not state the use of any on-board peripherals, such as GB-Ethernet, WiFi, or BLE). 7.6 W at 5 V is a current of 1.52 A.
Considering the data listed above it seems rather likely that a Pi 4 can be powered with a solid 2.4 A power bank (if no additional power hungry peripherals are connected) even though it is below the recommended ratings.