I posted something about this earlier today but I took it down because I needed to rephrase.

In the Sims 4 license agreement it states "You may not access, copy, modify, or distribute the Application, unless expressly authorized by EA or permitted by law." As we all know EA allows modding of TS4, that's not what I'm asking about, I'm interested in the next line. "You may not reverse engineer or attempt to extract or otherwise use source code or other data from the Application, unless expressly authorized by EA or permitted by law."
In the United States it is legal to decompile and reverse engineer code unless it is used to create a competitor project, then it violates copyright law. Under USA law it is also expressly permitted that you may decompile or reverse engineer code if for the purpose of creating compatibility between two codes/games/applications.

Would it be legal and within the limitations of TS4 license agreement then to decompile or reverse engineer code for the purpose of modding TS4? Since EA allows modding.

I'm not a lawyer or an expert on legal documents, but I would think so.... I think the methods on that might have limitations though... or rather, wouldn't be very user friendly... I remember someone on the forum talking about creating a trait only using Cheat engine and that was kinda frowned upon, seemingly unnecessary and not very user friendly in it's implementation.