First, let's try solving this as a standard cryptogram, assuming each 4-letter code represents a single character of plaintext. There are 16 distinct codes. For convenience, I'll assign each one to a single letter.
- TGGC = a
- THFI = b
- TIBC = c
- TIFB = d
- TIHC = e
- TJEC = f
- TJEE = g
- TJFF = h
- UACD = i
- UAEB = j
- UAIF = k
- UAJC = l
- UBAB = m
- UBDI = n
- UBFB = o
- UBGD = p
This reduces the cryptogram to:
gci oak hlakj. akncaf fc doag of akpknflcb obj fkee hka fhk pcjk mcaj: phoaf.
Sorting the distict ciphertext letters in descending order of frequency gives
akfcohjpbeglndim. English letter frequency order (according to the nearest e-book I have handy) is
EATOHNSIRDLMFUYWCGBPVKJZXQ. Zipping those two alphabets and using it as a translation mapping gives:
LOY HEA NMEAS. EAFOET TO UHEL HT EAIAFTMOR HRS TADD NAE TNA IOSA WOES: INHET.
OK, so that's all gibberish except for the
TO. I'll have to try a different approach.
The most common 3-letter word in English is
THE. So let's assume it's present in the plaintext, so either
obj and especially
gci can be ruled out as making the letter
E too uncommon. So that leaves either
fhk. So let's try each of these.
THE: --- THE --HE-. HE--H- -- -TH- T- HE-E----- T-- -E-- -EH --E ---E --H-: --TH-.
THE: --- -EH T-EH-. EH--E- -- --E- -- EH-H----- --- -H-- THE -TH ---H --E-: -T-E-.
THE: --- --E H--E-. -E---T T- ---- -T -E-E-T--- --- TE-- HE- THE ---E ----: -H--T.
The first two of these generate two-letter words ending in
EH, which is impossible (unless you count
MEH as a word). So assume the last case.
fc = a two-letter word starting with
T, so my original guess of
TO is probably correct. So,
-O- --E H--E-. -E-O-T TO ---- -T -E-E-T-O- --- TE-- HE- THE -O-E -O--: -H--T.
TE--. Note the double letter at the end. Probably
TELL. (Could also be the name Tess, but that's far less likely.) So
-O- --E H--E-. -E-O-T TO ---- -T -E-E-T-O- --- TELL HE- THE -O-E -O--: -H--T.
What about the longest word,
-E-E-T-O-? What matches that pattern, has its remaining 5 letters distinct from each other, and does not contain the letter H or L? I can find multiple matches in my dictionary, but they all end in
-O- --E HI-E-. -E-O-T TO ---- -T -E-E-TION -N- TELL HE- THE -O-E -O--: -H--T.
The next-longest word is
-E-O-T. Note that the first and fifth letters are the same and the word does not contain H, I, L, or N. In my dictionary, I found only two words matching the pattern:
REPORT and RESORT. So
n is either
So that gives us:
-O- -RE HIRE-. RE-ORT TO --R- -T RE-E-TION -N- TELL HER THE -O-E -OR-: -H-RT.
We're getting close. Filling in the rest of the letters and making the assumption that a person using a female pronoun has a traditional female name,
YOU ARE HIRED. REPORT TO MARY AT RECEPTION AND TELL HER THE CODE WORD: CHART.
OK, so the cryptogram is solved, but that doesn't explain the original code. Why 4 letters of ciphertext for each letter of plaintext?
Khale_Kitha gives a potential clue by pointing out that RFC 4648 defines the Base16, Base32, and Base64 binary-to-text encodings. The codes can't be Base16 (at least not the standard version) because that encoding does not use the letters G-Z. I tried Base64 in an earlier version of my answer but got nowhere. So let's try Base32.
I didn't get anywhere by decoding Base32 into 8-bit bytes (even with the required
==== padding at the end), so I'm just going to decode into 5-bit sequences, recalling that A=0, B=1, C=2, ... Z=25.
- A = UBFB = [20, 1, 5, 1]
- C = UBGD = [20, 1, 6, 3]
- D = UAEB = [20, 0, 4, 1]
- E = UAIF = [20, 0, 8, 5]
- H = TJFF = [19, 9, 5, 5]
- I = UAJC = [20, 0, 9, 2]
- L = TIHC = [19, 8, 7, 2]
- M = TIFB = [19, 8, 5, 1]
- N = THFI = [19, 7, 5, 8]
- O = TIBC = [19, 8, 1, 2]
- P = UBDI = [20, 1, 3, 8]
- R = TGGC = [19, 6, 6, 2]
- T = TJEC = [19, 9, 4, 2]
- U = UACD = [20, 0, 2, 3]
- W = UBAB = [20, 1, 0, 1]
- Y = TJEE = [19, 9, 4, 4]
That's still gibberish, though. Maybe the first 3 values in each set represent a year (which would make them range from 1966 to 2016). The last one could be a month (1-8 = January through August), the day of a month (I'd assume December, based on the memo header), or the score of some kind of sports championship that happened in that year. I'm sure the “UK” is relevant somehow, but I'm an American, so nothing about these numbers stands out.