As previously mentioned, they say that Siborne's book has been in print in one form or another since its first publication. As you will see, they were probably not wrong!.
The first reprints to appear were an American version and a German translation, as Peter Hofschroer says "at a lower price". There seems to have been great interest in Prussia in Siborne's book due to his positive view of the Prussian's role in Napoleon's defeat.
A Fifth Edition was published in 1904 by A Constable of London. This seems to have been an exact copy of the fourth edition.
The reader needs to be very careful in chosing which verions to study either by PDF download or purchase of a physical book. Many descriptions of the books do not say what edition they are a copy of
Due to the age of digitization a large number of new versions of Siborne's books have appeared in the 21st century.
Most appear to be scans of one or other of the original editions.
The one exception to this rule seems to be the edition by Leonaur books. Published in 2015. These have been fully type re-set, making it physically easier to read. They are available in hardback or paperback in three volumes (just for convenience). They do not mention this in their introduction, but their tgext comes from the 4th (revised edition). Also, they do not include any of the prefaces, and they only have two of the 17 appendixes, but they have some extra maps and line drawings
by Digby Green 2017