Here are the proper steps and resources to locate this firmware file:
Websites like Remont-Aud.net or 4PDA often host split RAR firmware files for TCL models. You will need to search for "TCL LED43S4900 firmware" or "MS6486" (the common mainboard chassis for this model) to find all parts (Part 1, Part 2, Part 3, etc.) in the same thread. Download TCL LED43S4900 Firmware part03 rar
The resulting extracted file is usually named V8-MS6486xxx.pkg or similar. Place this file in the root directory of a FAT32-formatted USB drive, plug it into the TV, and initiate the update by plugging in the TV while holding the power button. Here are the proper steps and resources to