From Cosimo Lupo:
"the latest fonttools now supports L4 instancing, i.e. the final "level" of partial instancing, thanks to Behdad! After L1=drop axis, L2=pin axis at arbirary location, L3=restrict min/max range of an axis, with L4 we can finally change the default value of an axis. E.g. say you have a font where weigth axis has two Thin and Black masters, and you want to have a Regular as default weight, you can now do fonttools varLib.instancer FoldIt-VF.ttf wght=100:400:900. Designers have been asking for that for years, even though at times moving the default may well increase the file size. Other times it can be beneficial, say you have a font with intermediate masters for various weight, wght=400 (default), wght=500, wght=700, wght=900 and one only requires weights between 700 and 900, we can now request wght=700:700:900 (by moving the default from 400 to 700) and thus drop the deltas associated with 400 and 500 (with L3 this would have not been possible before)."
For anyone who has integrated fonttools instancing into their vf sales cart process, such as to offer sub-spaces of the full family design space for a discount of the full retail price, this should be very good news.