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.
The recommended current requirement for Pi4B is 3A, so 2.4A power bank is rather not enough, and the pi could be unstable on such power supply. You need a powerbank that can supply at least 3.0A, then it will be perfectly fine for powering up the Pi4B.