Sample size in scientific experiments generally refers to the number of observations being made. For example, for sample size of 4 (also referred to as n = 4) there would be 4 individual observations. These could be 4 different people, 4 different animals, 4 different cars, etc. It has importance in trying to evaluate your data on a statistical basis. The greater the number of observations, the greater is the power of the statistical tests. So doing statistics on 100 observations is infinitely more powerful than using only 4 observations.