HHdbIV contains considerably more information than HHdbIII. Apart from over 8000 extra studies, also the solutions of tens of thousands of studies were updated, e.g. by adding sublines from primary sources. For instance, all sublines from the gigantic book 1414 Fins de Partie (1950) of Henri Rinck were added, while in HHdbIII in most instances only the main line was given. That is useful, because e.g. CQL allows to search for patterns in sublines.
Also, many endgame study enthusiasts reported thousands of cooks in studies to Harold. Even specially dedicated software was developed to systematically check every sub-7-man position in the corresponding EGTB, and the results were kindly supplied for implementation in HHdbIV. Probably all incorrections (where Black is able to refute the stipulation) in sub-7 man positions are now included in the database. Of course, unsoundness of other endgame studies is an ongoing issue; i.e. work in progress!
A major improvement in HHdbIV is that also the sources of corrections, modifications or versions are added as text before the 1st move of the solution. The same goes for the cooks, including the names of the person(s) who cooked the study (and the relevant move is marked with his initials). This information is relevant for people writing about endgame studies. Without HHdbIV it is almost impossible to find out where a certain correction of a study was published, or who managed to cook that famous prize winner!
More new details/corrections:
Similarly to HHdbIII, codes are included to be able to identify cooks, corrections and other relevant properties of the endgame studies:
@1: second solution (at move 1).
@2: cook; extra solution after move 1.
@3: incorrect; White is unable to fulfil the stipulation (in a win study Black draws or wins; in a draw study White loses).
@4: "super-cook" White can even win in a study with a draw stipulation.
(@1) or (@2): minor duals.
@1?, @2?, @3?, @4?: suspicious, or claim without analysis.
(c): correction; i.e. original study was incorrect.
(m): modification; i.e. original study was correct, the improvement has another motivation.
(v): version (perhaps a correction or a modification).
(s): corrected solution (without changing the position).
(ea): too many composers' names to fit. All names given as text before the first move.
(tw): twin study (also triplicates, quadruplicates, etc).
(pl): plagiarism or accidental re-composition (it does happen!). Although there is no doubt in many a case, all studies in the database are only considered "suspect".
(ph): posthumously published.
(te): theoretical ending (i.e. probably not an endgame study).
(cr): colours reversed (the original stipulation was, for instance: Black to play and win).
(ce): computer ending (EGTB-derived ending).
(tt): theme tourney.