How do the fields of medicinal chemistry, pharmaceutical chemistry, pharmacy, and pharmacology differ?
I'm in a medicinal chemistry department, the floor below is the pharmaceutics department. We are both located in our university's college of pharmacy. The pharmacology department is under the college of medicine in same building as biochemistry, microbiology, etc. There is little interaction between departments here and that's not good.

But we can break the differences in each discipline down into some order based on drug development. Medicinal chemistry and pharmacology come first, with the discovery of molecules that bind to receptors and other proteins and have biological effect. This used to be based almost entirely on natural products, and those still play a large role, but high throughput screening and computer modeling are becoming more important for discovery of drug candidates. While pharmacology focuses on the effects of drugs and other molecules on cells and the body, medicinal chemistry has traditionally focused on the chemistry behind them. A medicinal chemist might look at a natural product with desirable activity, and ask "how can I make this better?" and apply organic synthesis to produce new versions of that drug, with better activity, reduced toxicity, and longer half-life. Of course the biology cannot be ignored, and has become more important in recent decades. Our department used to be pure synthesis, but now only 2 of our 8 labs are pure synthesis, and 2 almost entirely biological.

Once the medicinal chemists have developed a drug candidate to the point they think it can be used in the real world, it moves on to pharmaceutical chemists. They not only have to refine the synthesis of the drug to produce it efficiently and reliably on large scale, they have to develop the formulation the drug is delivered as. If it's a tablet, they have to determine what to make the bulk of the tablet from, how to press the tablets, how to store them, etc. A large part of pharmaceutics is stability, how long can the product sit on a shelf and not degrade? How does temperature and humidity affect degradation? What does it degrade into and are those products dangerous? Pharmaceutics is also concerned with drug absorption and excretion. They often try to develop "extended release" formulations so that a tablet takes a longer time to break down in your gut, releasing the drug over a longer time and keeping an effective concentration of drug in your blood longer, thus requiring fewer pills and less variation in concentration. Pharmaceutics is a lot of organic chemistry, physical chemistry, analytical chemistry, and quality control.

Pharmacy is the end product of all of these. After a drug is developed and approved the pharmacist is responsible for distribution. While doctors prescribe drugs, they usually have very little training in how the drugs work and interact. The pharmacist needs to know more than just how to sell pills, they must have at least a working knowledge of the pharmacology behind the drug, be aware of potential interactions and side effects, and advise the patient on the best practices for using the drug. They must also be aware of the rules and regulations, and make sure the controlled substances under their care are not stolen or lost or abused. Substance abuse is actually the biggest workplace hazard for pharmacists.

I have some knowledge on these things because I'm in a med chem department, my father is a chemist for a pharmaceutical company, and my brother just got his pharmacy license, so we see all parts of the process.