Which transformer

How can I compute which transformer I need for a K4004B/K4006 with 4x8ohm or 4x4ohm speakers?

4x 8ohm = 2x 4 ohm (parallel)

4x 4ohm = 2 x 8 ohm (series)

It is impossible to have 4x4ohm parallel?

In 8 months extra space is created which requires 4 extra speakers. Can I connect 8 speakers? If so, which transformer do I need then?

4x 4 ohm is not possible.
Minimum impedance per channel = 4 ohm
You will have to create series and parallel circuits to achieve no less than 4 ohm.