What are the four requirements for a utility patent?

Utility patents are granted for inventions that can be described or categorized as:
-Machines (i.e. a device that accomplished a specific task -e.g. an engine)
-Articles of Manufacture (i.e. a single object without movable parts or equivalent -e.g. chair, pencil)
-Processes (i.e. a method of accomplishing a result through a series of steps involving physical or chemical interactions).
-Compositions (i.e. a combination of chemical or other materials).
-New Uses of machines, articles of manufacture, processes, or compositions.

The four requirements for a utility patent are:
1. The invention must fall within one of the statutory classes
2. The invention must be useful.
3. The invention must be novel.
4. The invention must be nonobvious.

By statutory classes it is meant that the patent is a machine, process, manufacture, composition, or any new or useful improvement of these. The USPTO regards an invention as useful when it is capable of accomplishing the task it was designed for (i.e. the invention must work). Thus, when filing a patent application it is important to demonstrate that the invention works as intended and therefore has utility. Novelty is a statutory requirement of the US Patent law. By novel it is meant that it differs in some specific way form prior art (i.e. existing knowledge in the public domain or previous patents). Finally, the invention must not be considered to be obvious by people trained in the art.

References:
[1] Stim, R. "Intellectual Property. Patents, Trademarks, and Copyrights" West Legal Studies.
[2] Black's Law Dictionary 5th ed., (West Publishing, 1979).
[3] Manual of Patent Examining Procedure, 8th Edition