A breve (double whole note) lasts as long as two semibreves. It fills a whole bar in 4/2 time.
The breve
These are three recognised methods of showing a breve but the one on the left is to be used for examination purposes.
The breve rest is shown below. It is used for an entirely silent bar in 4/2 time.
The breve rest
An example of 4/2 time is shown below.