A number of tools are available to systematically count the number of outcomes :
The tree diagram:
A diagram in which all options are shown as branches from a starting point.
This tree diagram has two "levels". The first level has branches and the second level has branches.
Writing out the possible outcomes systematically:
There are six ways to get two "heads"
(H) and two "tails" (T) when tossing four coins: HHTT,
HTHT, HTTH, THHT, THTH, TTHH.
A grid:
This grid shows the equally probable outcomes when tossing two coins.