The AP1120 is available in two flavors: Cornet-IP (also known as "HFA"), and SiP. The HiPath 4000 V3.0 supports both, but the Cornet-IP version, which is basically H.323, is the most commonly-used. The AP1120 will support one or two analog ports.
The "Cornet-IP" AP1120 will emulate one or two IP/HFA phone(s) e.g. optiPoint 420; therefore you need a HG3530 gateway. Use SBCSU to configure up to two unique stations on the HG3530, just like you were configuring regular IP/HFA phone(s).
To configure the AP1120, you must use "UMN" software (Unit Manager Network), which can be downloaded from the Siemens Software Supply server (SWS) or perhaps the SEBA portal. You can also fetch it directly from the manufacturer's website. Click the provided link for a PowerPoint slideshow that should help you with the AP1120 setup.
Essentially you will build the AP1120 to register as the one or two new HFA stations that you previously built on the HG3530. You then connect the analog phone(s) directly to the AP1120, and when you go offhook on an analog phone, the AP1120 emulates Cornet-IP/HFA signaling to the HG3530, which draws dialtone from the HiPath 4000. The AP1120 converts the proprietary Cornet-IP packets into analog, and vice versa.
If using the analog port(s) for FAX or modem, please re-post later, preferably after the AP1120 can provide dialtone to an analog phone. At that time I can provide a few more hints.