Hi, williamseal,
In 12 ac, wet becomes set.
In 18ac, area becomes urea.
In 27dn, bowl becomes boil.
Yes, in 1dn the correct letter is P: jokey b comes pokey.
The three examples of 4dn are given by the correct letters of 1ac to 20ac, 22ac to 37ac, and 1dn to 31dn.