Meaning of four/five digit/letter codes on passport stamps

It's a control number identifying the stamp for the purposes of detecting and deterring forgery as well as to identify the officer who placed the stamp in the document should that prove necessary.

As an example, here is an excerpt from the Schengen Practical Handbook for Border Guards (PDF) (emphasis added):

4.7 Different types of stamps are used to furnish proof of entry and exit (a rectangular stamp for entry, a rectangular stamp with rounded corners for exit). These stamps contain the letter(s) designating the country as well as indicating the border post, date, check number and a pictogram indicating the mode of travel used upon entry and exit (overland, by air or by sea).

Queries about entry and exit stamps, as well as documentation relating to forged, counterfeit, lost or incorrectly affixed stamps, can be asked to the contact point established by each Schengen State for this purpose.

4.8 Each border crossing point must keep record of the entry and exit stamps handed over to and returned by each border guard carrying out the checks. This will also include the reference of each respective stamp, which may need to be used for comparative purposes at a later date.

When the stamps are not used, they must be locked and access to them must be limited to the authorised border guards.

4.9 The security codes on the stamps must be changed at regular intervals not exceeding one month.