What does it mean by "Let her rip"?

In "four and a half minutes", why is there "a" even it has plural of "minutes"?

"A" is before "half" because a singular "half" is being referred to. Using "and a half", even when the word that follows is plural, is correct. This is how time and measurements are said in English. Ex: I will be there in one and a half hours! Ex: You will need two and a half cups of flour for this recipe.

Are "circumstance" and "situation" different? How?

A "circumstance" is the way that something happened or the things that affect a particular fact or event. Ex: What are the circumstances of trespassing on private property? Ex: Due to the circumstances beyond our control, we have to postpone the meeting. The word "situation" is different from "circumstance". A "situation" is a state that someone is in or the way something is positioned given the surroundings. Ex: It's an awkward situation. Ex: The girl is in a dangerous situation.

What does "spore" mean? Some kind of a virus or disease?

A "spore" is a small reproductive cell that could grow into something bigger. "Spores" are hard to see since they're so small! They're produced by fungi, plants, bacteria, and algae. Ex: I need to clean the mold in my bathroom. There are probably mold spores everywhere. Ex: You can see that there are spores under the plant leaves.

What does it mean by "Let her rip"?

This is an example of toilet humor. "Let it/her rip" is often used to mean "go faster" or to "continue/go ahead." However, it's also used in reference to passing gas (farting) or stool. Thus, "let her rip" here can be intended to mean "go ahead" and is also a reference to the waste that is passing through the tube. Ex: Let her rip! I heard this car's top speed is 200 miles an hour. Ex: Wow, you really let it rip. I can smell it from across the room.

Why "meet" is used, not "look, watch, see"? What are the different ways to use "meet, look, watch, see"?

"Meet" is used here to introduce a character to the audience. When someone is unfamiliar to others and you are talking about them or want to introduce them, the word "meet" can be used. "Look", "watch", and "see" would not sound natural to use in this context because they all involve physically looking at something whereas "meet" refers to simply greeting someone or becoming familiar with them. To "look", "watch", and "see" are used in context when telling someone to physically look at something and stare at it. Ex: Meet my friend Timothy! Ex: Look at that car, isn't it cool? Ex: Watch this! I'm going to do a backflip.