Many have criticized the new-generation BMW M4 for its controversial styling changes, but all is forgiven once you get behind the wheel and take it for a spin. The new M3 and M4 continue BMW M's reputation for building some of the best driver's cars on the market, but there's still room for improvement. Our spies have caught BMW testing a mysterious prototype based on the new M4 with some aggressive aero enhancements. This could mean BMW is cooking up a hot new M4 CS.

Alternatively, we could be looking at the long-rumored M4 CSL but CSL variants tend to have a prominent rear wing, which is missing on the prototype. Since our first sighting back in April, some of the camouflage has stripped away, revealing the car's Frozen Portimao Blue paintwork.

BMW has covered the front and rear with camouflage, but it's clear this is no ordinary BMW M4. At the front, the prototype is fitted with a prominent front splitter. There is also a larger rear spoiler and diffuser, as well as a titanium exhaust system and multi-spoke wheels with carbon-ceramic brakes.

Lowered suspension also brings the M4 CS closer to the ground. In its range-topping Competition guise, the BMW M4's 3.0-liter twin-turbo inline-six engine generates 503 horsepower and 479 lb-ft of torque, but the hotter CS variant could get a smaller power bump. The M4 CS based on the last-generation model had 29 more hp than the standard car, so the new version could have around 530 hp.

However, BMW's priority will be saving weight to improve the performance, so expect the new CS to feature more carbon fiber components. The rear seat will also be removed to save extra weight.

As for when the hardcore BMW M4 will break cover, rumors suggest the new M4 CS/CSL will start production in July 2022 and will be built in very limited numbers, but the prototype appears to be at an advanced stage of development so it could debut later this year. The M4 may not be the only model getting the CS or CSL treatment either, a BMW has recently been caught testing hotter variants of the M8 and M2.