Considering how the Sony Xperia Z1 is known as the SO-01F when it made its way to NTT DoCoMo, with the Xperia Z1 Compact being the SO-02F, logic dictates that the Sony Xperia Z2 and Z2 Compact ought to claim the 03F and 04F model names, respectively. Do bear in mind that these are all just speculation, since neither has been made official to date.
What we do know about the Sony SO-04F, however, it would measure 127.6mm x 65.5mm, and is a millimeter taller and wider compared to the Xperia Z1 Compact. At the back of this smartphone lies a 20.7MP sensor and Sony G Lens, which have always appeared on the Xperia Z range of smartphones.