So I've been testing and printing non-stop for the last 24 hours, and it seems like version 4.0 addresses pretty much every issue mentioned in this thread! Here's a quick summary of what I found
1) They now allow printing features that are smaller than the extrusion width. In the past, if you were trying to print a 0.3mm wall with a 0.4mm nozzle, it probably wouldn't work, but now you can use what they call "single extrusions" to print those thin parts. You do have to enable the feature first though by going to the Advanced tab and changing the "external thin wall type" to "allow single extrusion walls".
2) I was worried that this feature would only work if the size of the wall stayed constant, but it actually looks like the software is smart enough to taper the flow to create smaller extrusions in thin areas, or increase the flow to get thicker extrusions where needed. So if you have a wall where the thickness varied from 0.2 to 0.3 to 0.25 etc, the flow gets adjusted to try to create that exact thickness. Pretty cool!
3) There's also quite a few topics that I've seen about thin walls that had a gap between the perimeters. So if you wall was 1.0mm thick and you had a 0.4mm extrusion, then you might end up with a 0.2mm gap in the middle. Previously, you could use gap fill in most of these situations, but v4 lets you place a single extrusion in these thin gaps now. So it fills the gap in one straight pass instead of the infill style where it moves back and forth constantly. Just like #1, you have to enable this though by going to the Advanced tab and setting the "Internal thin wall type" to "allow single extrusion fill". Like before, it can also adjust if the gap varies in size.
4) There are a few options for customizing the single extrusions in the bottom left of the Advanced tab. If you're trying to print REALLY tiny parts, you might need to decrease the minimum extrusion length, since it seems that anything less than that value isn't printed. Mine defaulted to a minimum length of 1mm, and that worked fine for most of the parts I was printing. If you had a larger part and only wanted to print long single extrusions for filling in long gaps, it may make sense to increase this value so that you don't have to bother with really tiny extrusions (just focus on the big ones). So 1mm is probably fine for most, but maybe increase that value for larger parts, or decrease if you are printing really tiny < 10mm parts. The only other setting I had to play with was the endpoint extension. For one of the prints I was doing, it helped to increase this value slightly to 0.3mm. That seemed to help the ends of the single extrusions bond to the rest of the model better.
5) Finally, I found out there is also a setting that controls when perimeters are used vs single extrusions. If you are printing a real model, you're probably going to see perimeters for big features and then single extrusions for the smaller stuff. But it seems that you can actually adjust the preference by changing the "allowed perimeter overlap" setting. If you set that value to 100%, it would allow the perimeters on either side of your model to completely overlap before switching to single extrusions (so if you had a 0.4mm extrusion width, your wall would have to be less than 0.4mm wide before single extrusions). If you set it to 0%, then single extrusions get used the moment the perimeters on either side of the model touch (your wall would only have to be less than 0.8mm wide to use single extrusions). From my prints, normal perimeters tend to be a little stronger than the single extrusions, so I was keeping this allowed overlap around 30-50%. That seemed to create the strongest prints without allowing too much overlap.
That's a long summary, but I hope it's useful for someone! And thanks to the S3D team for adding the new features!!