Are there any significant post-processing benefits--in terms of the user work-flow and features--of migrating from Lightroom 4 to the its last stand-alone iteration (Lightroom 6)?
You can do a 30-day free trial and see if LR 6 is worth upgrading. You won't lose your LR 4 library (make a back-up, but I think Lightroom will do that, by default when it makes conversions to the database for LR6). As others have suggested, the benefits to some users may be tiny, but huge for others.
Anyway, given that LR Classic CC 7.2 does NOT have any focus stacking functionality (only Photoshop), I'm going to go out on a limb and assume that the stand-alone version of LR 6 also has no support for focus stacking.