Each of Vivaldi’s Four Seasons (Le quattro stagioni) is shown as being a part of two works - https://musicbrainz.org/work/7044d543-9f9d-4598-8ee7-0f9945d28595 and https://musicbrainz.org/work/87886dcf-9776-49cb-b6f5-10104da6e42c. This problem is referred to here - Levels in the structure of works - as an example of works with multiple parents. It seems to me that this is misleading, as the “Four Seasons” are just the first four concerti of “Il cimento dell’armonia e dell’inventione”. It seems to make more sense to me to have it structured so that the first four concerti form a sub-part of “Il cimento dell’armonia e dell’inventione” called “Le quattro stagioni”.
When the proposal here - Levels in the structure of works - is implemented, it might be that the larger work (Il cimento dell’armonia e dell’inventione) might be the subject of a “collection” relationship.
Any thoughts?
The “Four Seasons” work shouldn’t exist, because as you say, it’s just the first four concertos of Il cimento dell’armonia e dell’inventione. They’re linked thematically, but that doesn’t make them a “work”.
You can still add a “Four Seasons” Series though, to link them all.
They’re linked thematically and many sources consider them a separate work of its own that just happens to also be the first four parts of another work. Wikipedia/Wikidata, Allmusic, VIAF, and IMSLP have separate pages/entries for “Le quattro stagioni”/“The Four Seasons”, in addition to their entries for “Il cimento dell’armonia e dell’inventione” (for those that even have that). If we want to be able to say “this on MB is equal to that in Other Database”, we will need an entry for this in some way—either a Work (as now) or a Series as @HibiscusKazeneko proposes. However, additionally, many artists will perform “The Four Seasons”, not “the first four parts of Il cimento dell’armonia e dell’inventione” etc., and for Event set lists and Recording/Release-Work relationships, it may make more sense to do a simply relationship to “Le quattro stagioni” rather than 4 separate ones for each of its parts – in my mind, it’s somewhat similar to arrangements, in that it’s not just a one-off recording of those four sub-Works being performed together as “The Four Season” (or “Le quattro stagioni”), but something that happens again and again, which makes it worthy of being it’s own Work.
This is what led me to create “Le quattro stagioni” those years ago, after discussing it with @reosarevok (since I wasn’t sure of its appropriateness myself at the time).
Hmm. What makes a work? I agree with @Freso that
such that it is customarily regarded as a separate work.
However, what is wrong with it being “part of” Il cimento dell’armonia e dell’inventione, rather than link each concerto directly? That seems to me to properly classify it as a major sub-part of the larger work.
What, this one? 
None of what you say here make the Four Seasons concertos a “work” by any meaningful definition. You’re arguing that we should have an extra layer of works just because it’s more convenient to link one work than four. But even if your “Four Seasons” work was valid, a performance of all four concertos should link them individually. The “unit” work here is a concerto, not a series of concertos.
But it’s NOT a “major sub-part”. Really. It’s four concertos in a collection of twelve concertos.
One reason I ask is because the multiple parents were causing some problems with my Classical Extras plugin for Picard. However, I have now implemented a multi-parent capability which I think handles it (not released yet as I want to fully test it and there are not a lot of multi-parent works that I know of, so if anyone knows of some then let me know and I’ll add it to the test).
I think a good compromise seems to be to leave it alone. I didn’t realise it evoked such strong feelings.
In this case, the Four Seasons would be intuitively a collection to me, rather than a work, as well as Il cimento dell’armonia e dell’inventione. Each concerto is a work, and the four concertos would be part of two collections.
The Four Seasons and Il cimento… both respects the definition of a collection (I cite reosarevok from here):
Collection (Y part of collection: X, X collects: Y): wording could be changed but basically this would represent a case where the “parent work” is not necessarily meant to be performed as a set. For example, Brahms’ collection of 5 songs, op. 72, or Debussy’s second book of preludes, or Beethoven’s 3 piano sonatas, op. 10. Parts of a collection can still be ordered/numbered (and often are!) - it just doesn’t necessarily refer to the required performance order.
Op. 8 is a collection of pre-existing pieces put together most likely based on business decision by the publisher. First four concertos already existed before being published as part of this collection. Earlier hand written manuscript already collects these four concertos together. Later composer gives special group name for these four concertos by using Four Seasons as a title when dedicating op. 8 (first published version): “Pray do not be surprised if, among these few and feeble concertos, Your Most Illustrious Lordship finds the Four Seasons which have so long enjoyed the indulgence of Your Most Illustrious Lordship’s kind generosity”. Dedication also hints that count (dedicatee) had known manuscript copies before the work (or collection) was being published as part of op. 8.
We could argue about the definition of work. It all depends if we count collections as works. In MB we have commonly stored collections in work entities.
These four concertos are worldwide known as Four Seasons and different databases, libraries and stores typically include separate entry for this collection. For me it makes sense to store this on MB work entity when there’s separate VIAF code, Wikidata ID, IMSLP-page or basically separate entry on every service we know. It’s not only that we link to these services but many of them are linking to MB.
Composer himself did group these together and would feel silly to say “we know it better”. I see no harm done even if this might not be a work by all definitions. While academic scholars, printed scores, Encyclopædia Britannica, Wikipedia and hundreds of releases cite this being “the most famous work” of Vivaldi we could include it to our database by using the name composer himself gave to it.