Darrell flips a weighted coin 20 times and gets 4 tails. what is the experimental probability that the next flip will come up tails?

There is a fifty percent chance of the coin landing on "heads" each time it is flipped.

However, flipping a coin 20 times virtually guarantees that it will land on "heads" at least once in that twenty times. (99.9999046325684 percent chance)

You can see this by considering two coin flips. Here are the possibilities:

Heads, heads.
Heads, tails.
Tails, tails.
Tails, heads.

You will note in the tossing of the coin twice that while each flip is fifty/fifty, that for the two flip series, there are three ways that it has heads come up at least once, and only one way in which heads does not come up. In other words, while it is a fifty percent chance for heads each time, it is a seventy five percent chance of seeing it be heads once if you are flipping twice. If you wish to know the odds of it not being heads in a twenty time flip, you would multiply .5 times .5 times .5...twenty times total. Or .5 to the twentieth power. That works out to a 99.9999046325684 percent chance of it coming up heads at least once in the twenty times of it being flipped.