Sony was among the last few smartphone manufacturers to have compact-sized Versions in its lineup.
The company used to launch small-sized version s of its flagship with the Compact branding connected to it. Seems like things are going to change as none of the newly launched smartphones had a Compact model. We were expecting Sony to establish a Compact version as most of its new smartphones have a 21:9 aspect ratio screen which makes them taller and more difficult to interact with when using with one hand.
New rumours hint the Compact models will no more need flagship hardware but will probably be mid-range apparatus, one such example is your Xperia 4. Dependent on the rumours on Esato Forums and an image that compares the newest Xperia 4 to elderly Xperia Compact models such as the Xperia Z5 Compact and the Xperia XZ2 Compact. The alleged Xperia 4 is thought to be 65mm wide but is taller at 149mm since it has a 21:9 aspect ratio.
Most Compact smartphones were launched with the elderly 16:9 aspect ratio using an Sony Xperia XZ2 Compact being the exception with a taller 18:9 ratio screen.
The alleged Xperia 4 is thought to be 8.3mm thin but will miss out on a 3.5mm headphone jack. Rumours also point the Xperia 4 will be exclusive to NTT DoCoMo in Japan. If these will be accessible in different parts of the planet remains to be seen.