Shortly after officially announcing the Xperia Z4 and unveiling details of its flagship smartphone, Sony now seems to be getting ready to launch the Sony Xperia Z4 Ultra and Compact editions and appears to be testing the devices with different networks.

According to documents related to a couple of unannounced Sony phones that recently showed up at the Indonesian regulatory site Postel, the model numbers indicate that the devices could be Xperia Z4 Ultra and Compact. So far, Sony has not revealed anything about the upcoming devices, but the data on a Chinese blog shows that the Japanese electronics giant could be actually working on the Sony Xperia Z4 Ultra (C6800 series) and the Xperia Z4 Compact (D5800 series).

As per the data spotted on the Indonesian Telecom regulator's certification program, it is believed that the Sony E5803 model number corresponds with the successor of the Z3 Compact D5803 and is likely to be named as Xperia Z4 Compact. Similarly, the E6683 and E6883 model numbers could be the successors to the Xperia Z Ultra C6683/C680. This is based on the trend followed by Sony of keeping the same number for its devices, but iterating the letter at the front as its range evolves. The Sony Xperia Z4 Compact has also been spotted in a leaked accessory catalog for the spring/summer portfolio of NTT DoCoMo in Japan.

There are no technical details presently available for the Xperia Z4 Ultra and Compact, though reports suggest that the Xperia Z4 Compact could sport a 4.5-inch Full HD display and will be powered by a Snapdragon 810 processor. On the other hand, the Xperia Z4 Ultra is expected to feature a smaller 5.7-inch QHD screen and is likely to be powered by a Snapdragon 810 processor, paired with 3GB of RAM and a large battery. This compares with a 5.2-inch full-HD display and 1,920x1,080-pixel resolution in the Xperia Z4, which is powered by Qualcomm's 64-bit octacore Snapdragon 810 processor paired with 3GB of RAM.

Stay tuned for the latest rumors, specs and release date for the Sony Xperia Z4 Ultra and Compact editions.