What is written on the 'L is real 2401' plaque?

TL:DR: It's just a joke put in by programmers. It's believed to say just that; "L Is Real 2401," but it likely says "Eternal Star" since that is what the statue is.

Many believe is does in fact say "L Is Real 2401" (not 2041 as you wrote). It's led to many rumors that Luigi is in Super Mario 64, which has been debunked. This same plaque makes an appearance in The Legend of Zelda: Ocarina of Time as well.

One source also shows that someone once mailed a letter to Nintendo asking about this mystery to which they were basically told in response that it is a joke the programmers put into the game.

An Easter Egg Wiki for games discusses this more. Realistically, the plaque probably says "Eternal Star" which would make sense given what the statue is.

Luigi was suppose to make an appearance in Super Mario 64 as a second player, but was cut out due to the limitations of hardware at the time.

This was put in by the programmers of the game as a joke. They wanted to see if they could trick players into trying to "figure out what it meant", despite there being no hidden meaning behind the message. There was even an official letter from a Nintendo representative explaining this fact. Source

This is even more evident when you realize that the exact same sign was re-used in other Nintendo games, such as Legend of Zelda: Ocarina of Time, as shown below. So there is no hidden meaning to this message.