In plus models is it true that you can only fly 400 ft?
I heard that the display on screen only allows this height and cant be overridden

It is not true. The FAA says you cannot fly higher than 400 feet, but the aircraft itself is limited to 1640 feet.

By law you cannot fly higher than 400 ft . There is a great app called Hover check it .

"I heard that the display on screen only allows this height and cant be overridden"

Total BS.
I set mine to 200 meters. I normally stay under 400 feet but if a big bird is flying at me I need to be able to dodge him.
Best way to dodge a bird is straight up, because they can't make that maneuver.

As others have mentioned, there's a FAA restriction at 400 ft, but it's not built in the RC. The max height for the P4 is 1640 ft, or 500 m.

How high you can fly is determined by a) the max height hardcoded into the drone, which is 500 metres above your take off altitude, and b) whatever country you live in and its regulations. If you live on a desert Island which has no aircraft flying anywhere near it or you live in a zone where all aircraft are banned from flying, you could fly at 500 metres, unofficially, but in a case like that you would need to risk asses during your flight and probably best to use a spotter, so while you are looking at your RC tablet/phone screen, someone else is concentrating on looking for low flying objects. You could suffer a bird strike even if aircraft are not allowed to fly there.

I would suggest you check your countries legislation regarding operating a drone as a hobby user, and or a commercial user.