GTS-based prototype spotted sporting aggressive aero and a massive rear wing. But what on Earth is it?

A beefed up BMW M4 prototype has been spotted this week at the Nurburgring, hinting at an even more focused edition of the Bavarian performance coupe before a new generation arrives in a couple of years time.

This particular vehicle appears to be based on the already-wild M4 GTS, but turns the dial up to 11 with race-inspired components for the front splitter, side skirts, rear diffuser, and a massive fixed rear wing. There's also some vent-like structures on the front wheel arches, and the side gills are much larger than the GTS – very racy indeed.

While our spy partners suggest this could be dubbed the M4 'CSL' if it makes it to production, the extensive aero package leads us to believe this could be something else – perhaps a 'GT4' or 'GTR'.

There's been rumours of an M4 GT4 for some time, further fuelled by the fact BMW recently revealed the M4 GT4 race car last year – this prototype could be an early version of an upcoming road-car equivalent.

When the factory-backed Australian version of the race car was revealed in December, BMW claimed around 80 per cent of components are shared with the road car.

In saying that, this vehicle could just be a development mule for the race car – we'll have to wait and see if this spawns a production model.

The last time we saw an M4 prototype that looked like this was August 2016, though that vehicle was based on the 'standard' M4 rather than the GTS.

Should a 'GT4' make it to production, we'd wager that a small power bump over the GTS and less weight would be the likely changes other than the obvious aerodynamic upgrades, making it a track-ready alternative to the Porsche 911 GT3. Head to the galleries section for more images.

What do you think this M4 mule is previewing?