To use Apple TV 4K in S.Korea

You can use a different power cord (with a different wall plug), as that is a separate part that can be exchanged. The internal power supply can handle 220 V and 60 Hz.

As Apple doesn’t sell the Apple TV in South Korea, there will not be much services you can use there, as many content providers will lock out out-of-region access, and there isn’t a local tvOS App Store.

You won't be able to access your US account from another location, that is only valid for the US. Services will also lock out of region access and attempting to access content from other regions would be in direct violations to the terms of use for your Apple account as well as those services.