That's Life Competition No. 38

 1.     Imply
 2.     74
 3.     School Teacher
 4.     Ocarina
 5.     Durian
 6.     Behind
 7.     Adaptor
 8.     Favour
 9.     Write
10.     Mezzanine
11.     Mist
12.     Pantry
13.     Refurbish
14.     Mini
15.     Rift
16.     Pansy
17.     Emperors
18.     Builder
19.     Maroon
20.     Exclaim
21.     William Shakespeare
22.     Somnambulist
23.     Pillows
24.     Snore
25.     Pyjamas
26.     Midnight
27.     Insomnia
28.     2
29.     Palette Knife
30.     Foxglove
31.     Hallway
32.     Marigold
33.     Dragon
34.     Halloumi
35.     6
36.     Elk
37.     Only Fools And Horses
38.     Observatory
39.     Down In The Dumps
40.     While
41.     Wood
42.     Vermouth
43.     8
44.     Batman
45.     Yap
46.     D
47.     Grain
48.     Denise Richards
49.     Meets
50.     151
51.     Emerald
52.     Plan
53.     1B 4C
54.     Money Bag
Pet in a Puzzle (Page 19)     4

I'm probably missing something really obvious, but can anyone tell me the difference between 1D and 4D for puzzle 53?