The Z2 is very much on the schedule and BMW are aiming this car to be in it's most simplist form , Power from only four cylinder engines including a range topping four cylinder twin turbo as mentioned before on the cards but never divulged further.
The Z4 will eventually be offered with this engine later in it's lifetime but for now the only four cylinder being placed in the Z4 is the diesel as found in the BMW 123d , this is expected to be available after launch at the IAA next September . The petrol powered turbo charged engine currently undergoing evaluations in the 1er Coupe is expected to replace the mid level non-turbo charged six cylinder engines as BMW work forward to further increase efficiency without compromise.

As for the Z2 if you think back to the introduction of the Z3 you get the idea and the proportions expected fro the smaller BMW Roadster . Officially considered as the Roadster for the Premium compact class.
Again a Roadster in the classic sense and not seen as a baby Z4 , Z2 will use different ideas but still recognisable as a Z car.
To keep weight down BMW will stick with a simple soft top originally intended for the new Z4 , magnesium engine blocks with welded aluminium front frame will provide chassis engineers with a significant base to begin working on tuning the car to it's optimum requirements.

A Hybrid sportscar for everybody?
Having learned some ideas from the MINI-E Concept there is always a "What If" if it was applied to a BMW ? Although never seen before but BMW have developed electric cars on it's models for some time now and engineers have built for evaluation full hybrid models of the latest cars such as the 7er , 1er Coupe , X1 and X6 .
When Tom Purves suggested an Electric Rolls-Royce he was not just speculating , it has been trialled in both full or mild hybrid.

With the Z2 BMW want to be the first amongst Premium manufacturers with a full hybrid sports car either in Coupe or Roadster form but not as a super expensive concept turned reality but the first identified as a sports car for everybody. When you look at how Toyota caught everyone napping with the Prius and look how such cars from the US domestic manufacturers are still effectively showing Concepts and Pre-production models where Toyota are about to launch the 3rd generation of Prius.

BMW aim to offer something without reverting to the traditional hybrid concept of a sleeker shape to help with aerodynamics and increase efficiency . Such aids used for this purpose will include
BMW's active aeroDYNAMIC technology which also includes special aerodynamic wheel designs.

Of course this project falls under "Efficient Dynamics" as do other ideas such as "Active Hybrid" And "BMW eDrive" starting with the next generation of M5 a new function "M:e" which will not only close down cylinders but will also activate the technology that will make BMW M division the more forward thinking and revolutionary performance cars in it's segement.