It is entirely possible that one particular 4-cylinder (MINI Cooper S with ~180hp) has more power than some other 6 cylinder (VW VR6 12-valve 172hp), depending on the age of the vehicles, and other factors. However, as a really broad generalization, you should expect a given car with a 4-cylinder to be lighter, have fewer parts, and be more efficient, while a 6-cylinder version would be more powerful. Many modern 4-cylinders make more power than 6-cylinder engines did a decade ago, so don't be convinced you have to have 6 cylinders to make a car decent. Drive both and see what you prefer.
Another way to compare engines is based on the volume the cylinders displace. The displacement volume is often related to the power output. These are typically numbers like "2.0" or "3.6 Liter".
When you get right down to it, none of this is really as relevant as how much power, how much torque, and how much gas mileage you get from each engine.
Another adage to keep in mind is: "It is more fun to drive a slow car fast, than it is to drive a fast car slow." Grammar notwithstanding, it is very true.