Previous replies focused on ACID properties, with this post I observe other kinds of differences:

Advantages of Perl Hash:

• Easy to store a structure as value
• Easy to later add more hashes (databases) , but in db4 you commit on it ahead of time.
• Easier to store multiple values for a key (arrayref)
• No need to worry about recovery and proper shutdown

Advantages of BerkelyDB (not the same as DB_File):

• Easier to access values bases on combination of keys and values
• Can to construct cursors (iterators)
• Can construct custom indexes (not yet supported in the DB4 Perl api)
• The first 4k of data are in memory, the rest on disk
• Can add callbacks to modify the standard access and retrieval methods.

Comparisons with SQL databases miss the whole point of an embedded database. DB4 is not a multi-process system; it is an embedded database, a a C library intended meant to run in the same address space with the main program (when not using rpc). The general advantages and disadvantages of stand-alone models versus the client-server model should be noted.