It is puzzling that the customer care rep said you have 4 remaining device activations on your Adobe ID, but the 3M app said you reached the limit.
Do you have a second Adobe ID or perhaps a 3M ID that you have used in the past? Adobe supports account joining, which allows you to activate a device with multiple IDs (Adobe or third-party), so that you can read books purchased under multiple accounts on a single device.
It's possible that your Adobe ID has 4 remaining activations but that it is joined to another account which has 0 remaining.
You can check if your Adobe ID is joined to another account at Adobe Account Synchronization.
If you do find a second account, you might be able to work around this by having a customer care rep reset the activations for the second account.