Insert values from one column depending on value in another column

[Supersadie]

I have a list of values in columns A and B. There is a list of 'trigger' values in C, dictating whether A values or B values go in D.

The rules are:

1 Insert in column D a value from A when value in C reaches 4

2 Continue to insert in column D value from A until value in C reaches 5

3 Then insert in column D value from B until value in C reaches 4 again - return to rule 1

Many thanks to all.