At first, these are not greeting words. 'zao shang' -> 'morning', 'wan shang' -> 'evening', 'zhong wu' -> 'noon', 'xia wu' -> 'afternoon'. 'shang' in this word is '上' means 'up', 'wu' in this word is '午' means 'noon'. Generally, in Chinese, you can't tear the word apart to single characters and try to understand the meaning of the word.

Hello, So, I'd like to start with word "noon", which means midday. "wu" is just the same meaning in Chinese. Usually, " zao shang" is a little earlier than "shang wu", which means early in the morning. And "shang wu" means before noon. And "shang, zhong, xia" means "before, right and after" In the case "zao shang" and "wan shang", I'm not so sure the meaning of "shang". Sorry for this.