Enter data from one or other column (A or B) depending on value in C

I've been searching for an Excel formula to do this but suspect only VBA will do it. Can anyone help me please?

I have columns ABCD.
A and B contain data
C contains numbers 0-9 - randomly - sometimes there will be duplicates
Rules:
Enter data into D from A until number in C=4 (trigger value)
AFTER this point, begin inserting data from B
When data in C=5 (trigger value), AFTER this point, begin inserting data from A again.
Example follows. NB Data between trigger values will continue to be taken from either A or B until the next trigger.
Many thanks.

A AA 7 A
B BB 2 B
C CC 5 C
D DD 5 D
E EE 6 E
F FF 5 F
G GG 4 G
H HH 4 HH
I II 3 II
J JJ 2 JJ
K KK 6 KK
L LL 2 LL
M MM 3 MM
N NN 3 NN
O OO 4 OO
P PP 5 PP
Q QQ 5 Q
R RR 5 R
S SS 4 S
T TT 7 TT
U UU 6 UU
V VV 5 VV
W WW 4 W
X XX 3 X

Chirayuw.
You are truly a formula ninja!
So many have tried and failed on this one. That's brilliant. Thanks so much for your help.