curb adapter for this unit. i'm installing all new rooftop supply and return connections with all new ductwork

HVACDirect lists the appropriate accessory as the Roof Curb for ACiQ & Carrier Packaged Units – 3 to 6 Tons, model CRRFCURB001A01, which is the curb you should use for the 4-ton AC-I-48-INV-HP-PKG.