Basically, the "Four Corners Rule" means literally that the language inside the four corners of the contract will control the outcome of the dispute and outside evidence will generally not be considered. Now, there are exemptions, as there is in every aspect of the law, but when dealing with a breach of contract case, one should always turn to what the contract says to the controversy at hand.
Going Outside the Contract
The next practical question I receive is what if my contract doesn't cover the dispute I am having with the other party. If the Court feels the dispute cannot be resolved by the language in the Contract, or it is ambiguous, then the Court will allow outside evidence to show what terms were agreed upon by the parties, or what the intent of the parties were to cover the situation. Such evidence may be correspondence between the parties before the contract was formed, or testimony of witnesses who were present to the negotiations etc.