Since there are no dominant skintones in the S1-S4 tones, the child will express a random skintone anywhere between the two skintones received from the parents. So if one parent is S1 and the other is S4, then the child can be any skintone: S1, S2, S3, or S4. In my first run of Pleasantview, Lilith (S1) and Dirk (S4) had 4 children. The first two were twins (S4 and S3) and the last two were both S3.
That is exactly right although when they have kids they will only pass on s1 or s4 even though they express s3. All the kids Lilith and Dirk had were s3 in my game but passed on s1 and s4.