The fourth set of Wing Chun is an intermediate set which prepares you for the knife, the stick and the wooden dummy. The other sets of the Wing Chun system do not provide all the necessary footwork for fighting. This set provides additional footwork.
The fourth set contains charging and retreating movements. It contains sliding steps which are useful for maintaining balance when you are pushed. It contains stances such as the forward stance (with 60% of the weight on the front foot) which people only associate with other styles of martial art.
The fourth set of Wing Chun is the hand version of the knife set. It is said that the knife is an extension of the hands, for this reason you have the fourth set. The fourth set does not merely mimic the knife set but contains many actions which are useful when you are facing an opponent who can kick.
Note: Whether the weapons are really extensions of the hands or the hand actions come from the weapons is debatable. For example, the Bong sau, a unique action to Wing Chun, is a very common action in Chinese weapons fighting. It occurs both in the sword and in the spear. It is likely that many hand actions were derived from weapon fighting techniques.
The fourth set also introduces the double palm movement known as Po Pai (holding a board).
The following techniques for dealing with kicks come from the fourth set.