In OB, you explained that the Singers have four sexes. I was wondering... Can the Singers have genders other than those four, like humans? Even as simple as just not going with male, female, or malen/femalen. Could a transgender Singer use their ability to shift forms to change their biological reality? And, finally, could a Spren be non-binary, if it wasn't personified in a typical male/female way?
In the cosmere as a whole, a person's perception of themselves has a lot of power over both their Spiritual and Physical forms. It is possible, with Investiture, to change their biology to match Cognitive perceptions--and while this could be easier for some races (like the Singers) it's not outside plausibility for any race.
There are non-binary spren, actually--and you should be meeting one important one quite soon in the books.