China Unicom is expected to launch the iPhone 4 in mainland China on September 16, according to a new report. Citing a “source from China Unicom,” Caixin reports that customers who want to buy the phone from China Unicom will need to order the phone online in advance of the launch day. In addition, since the Chinese iPhone 4 is likely to see antenna and reception issues similar to those seen in iPhone 4 units from other parts of the world,…