While a dedicated charger intended for RC use is a great option, one should also consider that a lab power supply with adjustable current limit can also be used. This is appealing as there are few 1000W+ chargers available.
For example, this should work or come close (50V limit): https://www.amazon.com/Volteq-Variable-HY5020EX-Voltage-Protection/dp/B00PXJHRKO
To charge a lithium based battery, just set the voltage to the desired final voltage (50.4V for 12S setup) and the current limit to something safe (in this case all the way up since it has limited capability). This is identical to the constant current - constant voltage charge that any Lipo charger does.
You could charge the whole pack as a 12S setup, which makes things simpler.
The downside here is that there is no cell balancing. Extreme care must be taken to ensure cells to not get mismatched as it could result in overcharge. I would recommend that they be monitored externally. An smaller RC charger could be used from time to time to do balancing if required.