Hello Simon,
According to me the below would help to access those apps from GW Hub :
In the Central GW Hub System,
1. Create RFC destination to ECC with appropriate User credentials.
2. Create System alias in SPRO which will point to ur ECC.
3. Add and Register those 4 Fiori services in GW Hub by going to /iwfnd/maint_service t-code.
Once this is done i think u shud be able to access those from GW Hub.
Regards,
Ashwin