I'm attempting a conversion of some of the stuff I'd created for Sims 2, but I run into this weird lighting situation and can't figure out how to fix it. I've played with bump maps and speculars (I'm going to redetail the textures a bit because the white, on which the problem is most noticeable, is a bit bleh), I've looked at tutorials, and I can't seem to figure it out. At first, I thought maybe it was to do with the shadow mesh, but the rug doesn't have one, so why is it looking so weird? This is my first anything for Sims 4 barring a few simple recolors, so I'm completely lost.

What tool did you use to make these? TSRWorkshop or Sims4Studio?
It looks to me that the specular or bump textures are the caus ebut I can't know for sure without seeing the package.
Have you made sure that the texture dimensions are a power of 2? E.g. 128, 256, 512, 1024
The actual floor shadow of the bench looks a little broken as well as the legs don't seem to be casting full shadows.

I used Sims4Studio with Blender, and yeah the dimensions are right. It seems to be having a problem with another mesh that also has a fair number of UV islands to it, which, believe me, is a pain in the butt to try to fix. Apart from this, I'm not having any issues converting my TS2 content to TS4, and I hate the problem with a passion.

I didn't realize I'd forgotten to attach the packages, so here's all the iron garden ones as well as the bookshelf that's giving me issues.

I've only managed to look at your bench package so far and it appears that your textures are missing the alpha channels they need.
What I think is your specular texture isn't setup correctly either. The alpha channel is usually a greyscale version of the Diffuse but yours is 100% white. The main texture is usually blocks of colour that dictate the type and intensity of the shine.

Interesting; when I open it in S4S, I see solid gray for two of the DSTs and a dark grey with texturing on the other. Would you recommend I switch to using the TSRWorkshop instead? And I'm not sure exactly what's involved in "the alpha channels they need" as I haven't had any problem before--it's meant to be solid with texture and not see-through on the seat.

EDIT: I downloaded TSRWorkshop and opened it there. Still no indication of wacky lighting to me.

EDIT 2: Thanks to this tutorial, I was able to get some of it fixed by changing the Phong to Painting shader on wooden items (which, by the way, doesn't support alpha PNG textures). However, the bench started as "instanced" and seems to think it's a plant as it has a material definition for dormant foliage. This was stupid of me as it breaks chairs and a few other things.

EDIT 3: It seems my problems are likely due to not using properly transparent .DDS images for bump and spec mapping. Solved!