Up to now I have done the following.
1. Done a tune up (Cleaned throttle body, injectors. Plugs and coils are good).
2. Changed gear oil as the mileage reached.
3. Scanned the TCM and there are no any DTCs. Also I have switched 4 TCMs and checked and still the same. So the issue isn't with the TCM.
4. Checked the valve body. All solenoids are ok and lines are clean as well.
Now pending works are to check the the torque converter and the gearbox. But I believe as 1st,2nd and 3rd gears are shifting around 2000rpm, the issue isn't in torque converter and the 4th gear must be slipping.