To edit 4K footage in Adobe After Effects, you will need to first ensure that you are using the latest version of the software, as previous versions do not support native 4K video. Once you have the latest version, you can import your 4K footage into the project by going to File > Import > File. You can then create a new composition by going to Composition > New Composition, and dragging your footage onto the New Composition icon in the Project panel to create a new layer. To ensure smooth playback, it is recommended to set the Comp Panel resolution to auto and use a magnification ratio of 50 or less. Additionally, you can use a video upscaler like EaseFab to convert your 4K footage to a format that is compatible with After Effects, such as MOV, WMV, or MPEG2. This will allow you to edit your footage smoothly and without any lag.