That's a good solution for parts that are only thin-walled. I think for parts that have thin-walls and thick parts, then the thick parts would get messed up.

True, you might try to find a happy medium that works for both...

I printed a part that was 4 walls thick exactly (.42*4). However when I printed it there was a slight gap in the middle so the two  walls on either side did not bond.

My questions is was this gap caused because the extruder multiplier isn't correct or does the slicer actually "mush" the layers a little bit together so the actual width of each thread would be something like 41mm?

You can use the extrusion multiplier to correct this problem if you don't care about the thickness being exact. I was printing some two walled parts and they were not bonding so I bumped the extrusion multiplier until enough plastic was flowing to get the sides to bond. Just a thought...

I loaded a test stl with 1.68 width into repetier and slic3r-ed it. There's a teeny gap (~ .05mm) in the middle. I put the same STL into kisslicer and there is no gap.

Thanks.

Look at the gcode preview in Repetier and see if it shows a gap there.

My questions is was this gap caused because the extruder multiplier isn't correct or does the slicer actually "mush" the layers a little bit together so the actual width of each thread would be something like 41mm?