Become proactive and read the actual law as published by the FAA. The only people that have a published height limitation are commercial operators working under Part 333 and 107. The amateurs and hobbyists are simply required to maintain safe separation from manned aircraft and to operate safely. Everyone is subject to the "careless and reckless operation of an aircraft" provisions in aviation law but a mandated 400' altitude limit, be it listed as MSL or AGL, is not part of the program.
Because someone said it on the internet does not make it true. Perform your due diligence to become accurately informed. I could post links to the law but you would not learn how to do things yourself if I did. I'll provide a hint and suggest starting with Part 336 and expanding from there. When you registered your model with the FAA you agreed at least twice in the registration process to remain below 400' but that is not part of published law.
Depending on the country you are in, and for this discussion I'll reference the United States, the altitude you can operate at is established by many factors, including the distance relationship with a certificated airport and manned aviation traffic. The so called 400' limit is not a limit at all, nor is it a law or regulation referenced in Title 49 of the CFR's. In no location of the FAR's is a 400' hard limit placed on sUAS or model aviation that blankets the U.S. Such altitude laws generally state you must remain clear of controlled airspace until granted permission to enter, notify an airport operator when planning to operate within "X" distance of the airport, and remain clear and not present a hazard to manned aviation. There are nuances that extend to specific conditions and locations but in general you are not restricted to an altitude <=400'. You are however, limited to operating within line of sight (LOS) as referenced in Part 336 of the CFR's.
If you intend to function in activities that have laws and regulations you must become conversant in such laws or a couple things will happen; you will deny yourself areas of flight that are perfectly legal to fly in due to ignorance of law, and you will violate laws you did not know existed. The FAA publishes several books to assist in self education. One is the FAR/AIM, another is The Pilot's Handbook of Aeronautical Knowledge, along with Aviation Weather, and a slew of other self training aids that can be accessed at the FAA website. You don't need to be limited to what someone says on the internet, you can read the laws as published for yourself.