Always install the latest, stable version of MongoDB. See MongoDB Version Numbers for more information.
See the following release notes for an account of the changes in major versions. Release notes also include instructions for upgrade.
See Changes in MongoDB 2.4 for an overview of all changes in 2.4.
For MongoDB 2.4.1, 2.4 refers to the release series and .1 refers to the revision. The second component of the release series (e.g. 4 in 2.4.1) describes the type of release series. Release series ending with even numbers (e.g. 4 above) are stable and ready for production, while odd numbers are for development and testing only.
Generally, changes in the release series (e.g. 2.2 to 2.4) mark the introduction of new features that may break backwards compatibility. Changes to the revision number mark the release bug fixes and backwards-compatible changes.
Always upgrade to the latest stable revision of your release series.
The version numbering system for MongoDB differs from the system used for the MongoDB drivers. Drivers use only the first number to indicate a major version. For details, see Driver Version Numbers.