The BMW Z4 successor, co-developed with Toyota, will also lose its steel hardtop to save weight.

BMW has reportedly decided to move away from steel hardtops for its next-generation convertibles.

The company will switch to a traditional soft top for both the next-generation 4-Series Convertible and the Z4's successor, which is being co-developed with Toyota, unnamed sources have told German outlet BimmerToday.

If true, the move could be focused on reducing curb weight to improve performance and fuel efficiency. It could also be related to the relatively large trunk volume filled by a hardtop.

The current drop-top 4-Series features a three-piece roof that opens and closes in 20 seconds at speeds of up to 11 mph. In contrast, the soft-top on the rival Mercedes-Benz E-Class Convertible can be raised or retracted at speeds of up to 25 mph.

Separate reports suggests the next-generation 3-Series, known internally as G20, will be lower and wider than the current edition. The front end is expected to bring evolutionary changes, while the overall body shape is said to adopt a more muscular appearance.

The next 3-Series is rumored to be on track for arrival in 2019, two years ahead of the new 4-Series Convertible.