Hi,
the licence should show up on your InMusic page of registered products. It is listed as Engine DJ Stems:
https://profile.inmusicbrands.com/customer/account/
If it is not there, something went wrong. Maybe you used a different email address? Since you have bought it, contact the support team, they can help you. I once had a similar issue with a plug-in from AIR, their support added it to my InMusic page. 
It seems that the Stems licence must be assigned to your account/profile, which you then use with your Prime 4.