The Custom Shop v2 is the new one. It does not support AT4.
AT4 users are encouraged to install AT5 CS. All your At4 gear will be available in AT5 CS and you will be able to Try new gear there.
From IK Multimedia's recent email:
AmpliTube 5 CS requires no additional purchases and will read all existing AmpliTube gear you own, along with any custom presets or other settings.
With this release, IK is ending support for AmpliTube 4 CS. We strongly encourage all users to upgrade to the new version for the best experience.
Similarly, since AmpliTube 5 CS offers the new Custom Shop 2, all future Custom Shop initiatives will only be available to AmpliTube 5 CS users.
AmpliTube 4 CS users will still be able to access and restore their gear, but will not be able to purchase new models or access giveaways like the current FREE SOLDANO SLO-100 promotion.