I am very curious how far the game can be modded? Is there a point where you hit a wall and cant make the game do something?

What do you mean by "modded"? Theoretically one can "mod" the game to the point of replacing it entirely .. the question is really, how would it integrate with existing code and how sensible would it be (I mean if you wanted a *completely* different game, the more reasonable thing might be to just play a different game)

So modding could mean different things for different people, but my definition I will use here is when you add files or alter non-binary (executable) files. I will also limit to only add/alter files in the documents directory and not the game files. So practically we are talking about package files or scripts.

So it is a bit unclear yet just what is available, what can be done etc. But to get a grasp of it we have to look at the big picture. When you run The Sims 4 you start the C++ frontend. This is responsible for loading all files and starting the Scalaform (flash) user interface (UI). The frontend is also responsible for starting the Python simulation engine and handles the communication between the simulation, the UI and anything else. This C++ frontend is, by my definition, not moddable.

The UI is made up of different Scalaform files that cover parts of the interface, changing these are possible but not very convenient. Only one mod for each part can be used and it needs to be updated with most major patches. It is currently unclear if new commands can be created (interactions between the UI and the Engine) or if a change to the C++ frontend would be required. Otherwise the UI should be able to be completely redesigned both in graphics, layout and functionality as the UI can be scripted.

The Engine controls the whole simulation, pretty much everything that happens in Live Mode. Here pretty much all sims behaviours and how the world works should be alterable. All Nraas mods should for instance be possible in some fashion. Here I am talking about the logic and not the visuals, I will not go into detail about clothes, hair and other such stuff as I have no idea how the situation is with those. The Engine is written in Python which means that any code can be replaced and altered and conflicts between mods can be avoided. What has not been thoroughly investigated (as far as I know) is what can be saved and loaded.

So I have mentioned Live mode, where almost anything can be modded (doesn't mean anything can be easy, or turn out good). Looking at Build/Buy mode we can change the UI, but most of the rest seem to be in C++ and therefore out of reach. Objects, recolours etc are defined in files loaded by the game, so those can be modded and created, but changing build tools is likely out of the scope of modding. Same if we look at CAS, here again the UI is in Scalaform but the rest is in C++, but we again can create content by adding files that the game reads. Here things like slider mods could be possible, but might also not be at all.

To conclude: Anything gameplay related should be moddable, but it is unclear if settings can be saved. Build/Buy and CAS are likely out of limits for major modding. UI can be altered to look and behave differently, but it is unclear if new actions can be created and compatibility is an issue.

Hope this helps and you will be inspired to test things out. As I am but a simple modder, I might have made some errors in my analysis, but I am pretty confident in the results.

This is a great analysis and i appreciate the awesome response! I much like others have had lots of ideas for custom content. I was just curious how far it could go. So again thank you!