Possible reasons:
* Low or dirty automatic transmission oil level: Inadequate oil level or quality can lead to slippage and loss of gears.
*Faulty Shift Solenoid: The solenoid responsible for shifting into 4th gear may be faulty or have poor electrical contact.
* Worn Friction Linings: The friction linings that transfer power to the transmission can become worn, causing slipping and loss of gearing.
*Damaged Gears or Shafts: Mechanical components such as gears and shafts can become damaged, resulting in loss of transmission.
* Problems with the transmission control unit (TCU): The TCU is responsible for controlling the operation of the automatic transmission. A faulty or incorrectly programmed TCU can cause shifting problems.
* Loose or damaged connections: Connections between the transmission, engine and control system may be loose or damaged, resulting in loss of signal or power.
Possible solutions:
* Check the level and condition of the automatic transmission oil and replace it if necessary.
* Check the shift solenoid for faults and replace it if necessary.
* Check and replace worn friction linings.
* Inspect and replace damaged gears or shafts.
* Diagnose and reprogram TCU as necessary.
* Inspect and tighten all connections.
Important:
*Do not ignore this fault as it may cause further damage to the transmission.
*Consult a qualified mechanic or auto repair shop for diagnosis and repair.