Question about wich elements are taken to the new generation

Hello,
while trying to learn about GA the following doubt appeared:

Let's say we have the following initial population

0001 = P0
1001 = P1
1010 = P2
1110 = P3

We rank the four with a fitness function

P0 = 19
P1 = 10
P2 = 15
P3 = 18

Now we chose the best for reproduction.

Let's say roullet picks P0 and P2. We cross them and get two new elements for the population.

Previous generation: P0 P1 P2 P3
New elements: P4 P5


The question is this:

* Do we keep crossing until we have a new generation of N elements (in this case four) making that all members in the next generation are completly new
or
* Do we take some of the elements in the current generation to the next one

Thanks for your comment.
.