Good job going ABM and MDM. However, you need to understand the purchase process to ensure automatic device assignment. Remember, ABM is all about asset chain of custody. This means the entire process of purchase must be within the links of trust (customer business entity > reseller (optional) > Apple).
If the customer bought via the retail channel ("directly from Apple and Apple's online store," or other retail channels like Amazon for example), then the assets cannot be automatically associated to ABM since chain of custody cannot be proven. If the customer is buying direct from Apple, they must reach out to the Business Sales team in the store. The customer will then be assigned an Apple Customer ID. This ID will be associated with the ABM and all future purchases made through the Business sales team will be automatically assigned. Going to apple.com or walking in a making a retail purchase is not eligible for ABM association. If the customer is large enough and they plan on purchasing at least $5000 annually, you can ask the Business Sales team to spin up an eCommerce portal. This is a custom Apple online store just for your customer. You as the consultant can be made a recommender. This will allow the customer to order without going to the store or calling a Business Sales associate.
If the customer wants to buy from someone other than Apple, they must seek a reseller who can participate in the device enrollment program. In this case, your customer provides the reseller with their organization ID (available in ABM) and the reseller provider the customer (or you) with their reseller ID (you must enter it into ABM to complete the link in the chain). This allows the reseller to assign hardware to the customer's ABM.
For the 4 iPads that have already been purchased, you will likely not be able to get Apple to backload them since I will assume these were made through the retail sales channel. However, all hope is not lost. For these three units, you can use Apple Configurator (on iPhone or Mac) and you can side load the units into ABM. Please note, side loading does offer a 30 day opt out period. However, few users will seek this. By side loading the hardware assets, you can manually inject the hardware into ABM, thus allowing automated enrollment from your MDM. Side loading will require you to erase the device. If you have already started using the 4 iPads, make sure data is backed up before starting the side loading process.
Pro tip: If you use Configurator on iPhone, you can select the MDM you want the device assigned to. If you add via Configurator for Mac you will need to log into ABM and reassign from "Configurator" to your MDM server.
Also, depending on your MDM, you may need to wait 15 minutes or longer (24 hours (looking at you Intune)) for the device sync to complete. In your MDM, check your pre-stage to visually confirm the device has been assigned. Once assigned, boot the unit up and allow it to auto-enroll.
Hope this is helpful. Happy New Year.
Reid