Converting Maxis Wallpaper from The Sims 4 to The Sims 3?

Well, I can tell you right now that you're going to be told that this isn't the right forum for your question. The Sims Resource has some tutorials here, but I can't seen any that are specifically for wallpapers. Mod the Sims does have a tutorial here. And here are a bunch of threads talking about making wallpapers. Here is TSR Workshop's page that talks about what textures TS3 uses. You can take a TS4 wallpaper diffuse and import it into TSRW as a non-recolorable overlay. Or you can desaturate a TS4 diffuse and import it as a multiplier. But, then you'd need to make a mask so that the wallpaper can be recolored. You can even make one swatch with a non-recolorable overlay and a second swatch that is recolorable. Also, the specular map that TS4 uses is different from what TS3 uses. TS3 speculars are greyscale. Basically, you take a TS4 diffuse, desaturate it, and darken or lighten the texture according to what kind of shine you want. Darker means less shine. Lighter means more shine. The normal map is made the same way as you'd make a TS4 normal map. No difference. You import the normal map in the meshes tab in TSRW. Take a look at The Sims Resource's tutorial page for more information.

Edit: Forgot to mention that TSRW uses DDS files (if I remember correctly) so export the textures from Studio as DDS files. You might need to resize the TS4 texture files. Use the TS3 files as a reference for sizing.