Is there a big upgrade from the S100? A Sony RX100 IV for example

I've been using the Canon S100 for a few years now. It has been a great little pocket camera and I still like it. However I'm starting to wonder if it is time to upgrade to a few more megapixels and a little more post processing "play-ability."

Would I notice a big difference moving to a newer pocket cam?

Dunno - is there anything about the S100 that's leaving you wanting more? Bear in mind that the RX100-IV you mentioned in your title is a tad longer, a tad higher, and 5/8" thicker - which will make it a less-than-ideal pocket camera.

Just curious if it is worth upgrading my 4 year old tech. I would love the ability to do more editing of the files. So often, circumstances I'm using this ca,era in are less than ideal, so some post work is almost always necessary.

Well, the biggest difference you might see is with the bit-depth of the RAW files. Most Canon RAW files allow 14 bits/pixel (14 steps of tonal range). If the Sony offers more than that 14 bits, you'll get more steps, which may help in recovering shadows in low light. Ultimately, however, you reduce your image to the 8 bit tonal range of a JPEG. The other thing you might see with newer cameras is lower sensor noise at high ISO or long exposure. Against that, there's the consideration that smaller pixels are generally noisier than large ones. So try comparing some high-ISO shots from the cameras you're looking at with your S100.

If I were in the market for a high end compact camera, I would be VERY tempted by the Panasonic LX100! It has a cool EVF, a huge sensor, a great lens, etc...I've been tempted by this camera time and time again!