1-3.
Self-explanatory.

4.
K1 visa:
This section is intended to provide details about any previous marriages which must have ended by now.
Don't mention your fiance as your spouse as he/she is not your spouse yet.

K3 visa:
Include details about current marriage and any previous marriages.
4d: Write "Not applicable - applying for K-3 visa based on marriage to US citizen".

5.
List all your children here. Check appropriate box depending upon whether they are immigrating at the same time or they are following you a few months later.