The sentence that should be omitted in the interest of unity in paragraph 4 of "Successful Exercise" is:
"For example, a person who enjoys running may find it easier to stick to an exercise routine if they run with friends or join a running club."
This sentence deviates from the main idea of the paragraph, which is about the importance of setting realistic goals, developing a plan, and overcoming challenges. While the example provided is a valid strategy for some individuals, it does not directly relate to the main points discussed in the paragraph and can be omitted without compromising the overall coherence and flow of the text.