When you see the circles appear, immediately press the number key corresponding to the number of circles.
Subitizing is the rapid, accurate, and confident judgments of
numbers performed for small numbers of items [Wikipedia].
Judgements for item counts between one and four are roughly
constant (meaning we can "see" the count instantly without
counting), but after that, each additional item takes longer
(meaning we need to count).
Subitizing is evolutionary useful; for example, an animal may want to quickly check the number of chasing predators without needing the advanced mental capacity to count to large numbers.