The AP621 firmware is still part of the RFS4000 image, so if you install it, it will still adopt and operate the AP621, but you can't call support for any issues that might arise.
From the CLI, you can enter: show device-upgrade versions to see which APs are supported out of the box.
On the RFS4000 the included firmware is for the AP621, AP622, AP650, AP6521, AP6522, AP6532, AP6562. Any other APs require additional firmware to be loaded into the RFS.