The Difference Between 4- and 5-Blade Fans
The main difference between a four and five-blade ceiling fan is one of aesthetics and personal taste. However, the laws of physics do dictate a slight difference in performance between ceiling fans with different numbers of blades.
Noise vs. Efficiency
As a rule, as the number of blades goes up, the fan tends to be quieter while circulating less air. Additional blades increase the drag on a ceiling fan's motor and will slow it down. This is one of the reasons why industrial fans (like wind turbines) usually only have two or three blades. They can go faster and move more air, and noise isn't so much of a concern for them.
A Balanced Ceiling Fan
These days, many ceiling fans designed for the home come standard with four or five blades. Beyond having a pleasing symmetrical aspect, it's at the 4-blade and 5-blade level that the ideal balance is achieved between the amount of air circulated and amount of tolerable ambient noise.
The Bottom Line
That being said, you are unlikely to notice any major difference in job performance between fans with fewer or more blades. These days, advancements in overall residential ceiling fan design, balance, and low-drag blade shapes and pitch ensure that ceiling fans with a non-standard number of blades are still able to do their job well.