Question

a wave raises a buoy twice every second and the crests of the incoming waves are 4m apart
what is the wavelength in meters?

Answer 1

You just said that the crests of the incoming waves are 4m apart.

That's it ! That's the wavelength. The distance between crests. It doesn't matter how often they sail past you.

The question I was expecting here: How fast are the waves traveling ?

That's (frequency)x(wavelength). 8 m/s.