Overheating problems have many causes, as shown by the attached pages.
You need to narrow down the cause:

1) Does the 4th bar appear at all speeds, low speeds, idling  etc?
How about on the highway?

2) Next morning, before starting engine, open the radiator cap. Is
the coolant filled up to the cap, or is it below the neck?
Are there rust particles floating in the coolant? Is there any
oil mixed with water, fuel/exhaust smell?

3) Close the cap, run the engine, and observe the radiator fan action against
the temperature indicator. At what bar it starts, and is the fan speed
strong or weak? The fan must run on and off cycles within the 3rd bar.
Look around the hose connections for leaks.  Is coolant leaking under
the engine (possibly from the core plugs?)

4) Look at the water pump pulley - any slippage (screeching noise)?

5) Did you use tap water for the coolant ? Did you add the additive/chemcials
for corrosion protection?

6) Pull out the engine oil dipstick, is there any creamy brown contamination
of the engine oil?