Okay, here's the question :

Why we can party up to 4 people but the dungeon only allows 3 people to go in?

Then whats the point of partying up to 4 people?

It happened when we want to go to Angor Quarry, when the 4th people of my party wanted to go in, it said "The environment is full".

I dont know if its only happening to this dungeon, because i've only reach this point rite now.

This is because you make a party before entering the instance. Some instances are 3-player, some are 5, etc. When you make a party, the game doesn't know what instance you are planning on entering, so you can make a party larger than allowed. However, the quests for the dungeons do say how large a party may be.

Crap and needs to be changed!