I love plants, but I’m confused over the difference of these 4.
I’ll help you out (again). 
Botany: This major usually emphasizes identification of flora and fauna, and memorization of plant classification. That is not needed much anymore because there are apps you can take into the wild with you and identify anything you find by looking it up online.
Plant Biology: This emphasizes the lab science of plants, and is heavy on biology and chemistry and some biophysics and genetics. The goal is to understand all about how plants function, and this major is typically offered within a biology department (both undergrad and grad levels). Some courses may involve going into the wild to study plants, but the focus will be on understanding the plants themselves, often at a microscopic level. Majors in plant biology may share several science classes with pre-med majors before they get into higher level courses that focus only on plants.
Plant Science: This major emphasizes how to grow and use plants in the real world, such as for agriculture or horticulture. It may cover some genetics, physiology and breeding, but will also look at soils, diseases, insects and more. It is usually offered in a department of agriculture. Majors will have courses that give them experience not just in a lab, but also in a greenhouse and field. (Ecology is usually offered in either a biology department or an agriculture department, and also includes field work but in the wild rather than in a plot of land that has been planted.)
Biology: This is the study of all living things, not just plants, so it covers plants, humans, animals, etc. A lot of pre-med majors want this degree because it usually includes courses where you dissect things and learn about anatomy, and also where you learn about cancer and other health issues. Plant biology or plant science would not cover human health and epidemiology – instead it would cover plant pathology (diseases) and other topics specific to plants. You will find a lot of competition for grades within biology departments because the pre-med majors need very high grades to get into med school.
Thank you!@#!$
I disagree somewhat with the distinctions made by the other responder.
Botany is not limited to plant identification and classification. It not only includes that, but also would include various other subfields concerning plants as organisms (e.g., plant anatomy, plant physiology, plant ecology, etc.).
Botany is one of the traditional major subdivisions of biological sciences (the other being zoology). Free-standing botany departments are less common now since in more recent years, many universities now divide their biology departments into departments of molecular biology and departments of ecology & evolutionary biology (or, sometimes, organismal & evolutionary biology). Much of what previously taught in botany departments is now found in departments of ecology & evolutionary biology. In many cases, the coursework on botany is not covered as a separate topic e.g., plant ecology, but might be covered as part of a broader topic, e.g., general ecology, field ecology, etc. Some land-grant public universities are more likely to retain the traditional division into departments of botany and departments of zoology.
It’s difficult to make a distinction between plant science and plant biology simply on the basis of the title of a department or major. In general, they are equivalent terms most commonly used to refer to applied plant science fields as found in colleges of agriculture. In recent years, many colleges of agriculture have consolidated various separate departments of applied plant sciences (e.g., plant genetics/breeding, plant pathology, agronomy/crop science, horticulture) into broader departments of plant biology or plant science. In many cases, these departments also incorporate botany coursework rather than only the applied or strictly agricultural areas.
@zapfino Cornell, Minnesota and UC Davis all have Plant Biology in the Biology department and Plant Science in the Agriculture department, and I’m sure many others do as well, and the majors are quite different from each other. I did say it is “usually” done this way because of course colleges and universities can organize their departments and majors however they want, and give them all sorts of names.
Yep, they go by all sorts of titles and organizational structures. Too many students on CC only look at titles and fail to actually look at the course listings and major requirements, which is why I added some other distinctions. It doesn’t mean you’re wrong, but there are exceptions to the examples you listed as well.