Is it possible to upgrade MongoDB from 2.0 to 2.4 directly
In general, yes, you can do this, the 2.4 binaries can be used as drop in replacements.  However, there are exceptions.  In sharded clusters for example, you have to make sure to do the meta data upgrade 

Essentially, the caveats are all really deployment and feature (authentication), not version based.  Basically make sure to read the notes here:

http://docs.mongodb.org/manual/release-notes/2.4-upgrade/