if you want a single Subscription Key to be used for all the APIS, why cant you add all the APIS under one Product and do it.
Like in your Example.
Product 1 can have API1, API2,API3, API4 and Product2 has API3, API4
So that you can subscribe with one Subscription key with Product1. APIS can be subscribed to Multiple Products.
if you want to change the Subscription Key,Then you can use