We know M4 is a high performance version of the 4 series. What is the difference between these two cars?
Exterior:
The BMW 4 series and M4 have pretty similar exterior, but M4 has its exclusive M-sport line. This package will make it has more sport feeling.
Interior:
The Interior between these two cars just have a little bit small difference, the material of BMW M4 main control is made by carbon fiber.Carbon fiber is a flexible fabric-like material that, when combined with a polymer, can be molded into the shape of a car part that is stronger and lighter than today’s steel and aluminum parts. The higher cost is based on the fiber material itself, as well as longer production times. Metal parts can be stamped in seconds, but it can take several minutes for a carbon fiber part to be molded and cured. And also the seat of these two cars has some differences. The seat of BMW M4 is the sport seat which has more inclusion for people’s body. 
Performance:
When comparing these two luxury cars, the main difference between the 4 Series and M4 will come down performance.
The BMW 4 Series offers a 2.0-liter TwinPower engine that produces up to 248 horsepower and 258 pound-feet of torque. Other powertrain options include a 3.0-liter TwinPower engine rated at 320 hp and 330 lb-ft. Both engines have a twin scroll turbocharger that optimizes output on a relatively low tachometer. The BMW M4 is equipped with the 3.0 L M TwinPower engine, which produces slightly more than 440 horsepower and just over 400 lb-ft of torque. The engine uses a double single-scroll turbocharger to improve performance under harsh conditions. The BMW M4 comes standard with a six-speed manual transmission system. The BMW 4 Series features an eight-speed automatic transmission. Other options are provided in driving dynamics control. For example, you can choose “sport mode” to ride smoothly on the freeway. Both BMW models are equipped with automatic start-stop technology to improve efficiency in the case of heavy traffic congestion. In addition, 4 Series uses braking energy regeneration technology to improve fuel economy.