PBR is actually much simpler and easier to get good results with than how game engines used to work. There’s 3 main things you need to worry about, your base color, metalness, and roughness.
First thing you’ll want to know if a material is metal, because that will effect your base color, painted metal does not look metallic, rusted metal does not look metallic, most things are not going to be metal or look metal, so you can leave metallic at 0, you don’t necessarily need metalness texture if your texture is 100% metal or 100% not metal, just a constant 1 or 0 will do the job.
Next I’d look at the base color, here you will want to reference pbr charts, you can find them easily on google. Make sure you know if you are looking a metallic work flow charts or specular work flow charts. The metallic work flow puts the specular value of a metal material in the base color, where in specular workflows, they are their own separate textures. You should not plug anything into the specular slot in UE4, if you want a physically correct material. Make sure sRGB is checked on the base color texture, and leave it unchecked for metallic or roughess. Use PBR charts as a starting point, do not put any AO or lighting or highlights in the base color, it should be pretty flat looking.
Lastly is the roughness. 0 is perfectly smooth, mirror like reflections, I would avoid using 0 if possible because reflections often have errors, .1 is a good starting point. 1 is completely rough, not shiny at all, something like rust or an old dirty coin. You can also look at roughness charts for this, but it’s just as easy to guess and play around with. Roughness is where you are going to put a lot of detail and variation, if people touch your material, there’s going to be variation from people’s oily hands.
Of course you’ll probably also want to use a normal map, but there’s nothing new or different with PBR.
And there’s also custom hair/skin/cloth/clearcoat materials for those specific use cases.