The meaning of the term "humanist" during the Renaissance in 15th century Europe can be found in option 4, which states that it refers to masters who emphasized skills that were developed through discussion and debate. This option correctly identifies the essence of humanism during the Renaissance.

Option 1 states that it refers to masters who taught grammar and rhetoric. While the teaching of grammar and rhetoric were part of humanist education, it does not fully encompass the meaning of humanism.

Option 2 states that it refers to masters who taught poetry, history, and moral philosophy. Again, while these subjects were taught and valued by humanists, it does not capture the complete meaning of humanism.

Option 3 states that it refers to masters who taught subjects that were connected with religion. While religion was an important aspect of society during the Renaissance, humanism focused more on the study of human nature and the potential of individuals.

Therefore, option 4 is the correct answer as it accurately describes the meaning of the term "humanist" during the Renaissance.