Refers to one of the MULTIPLE meanings Q has layered into the mystery of D5 which Q mentioned in a few drops.
In this case, the letter, dated 4 May, points to one of those meanings:
4th day 5th month
4 5
D 5
Not sure, but I bet Praying Medic will steal an anon's brilliant insight and post it as one of his own "decodes" as soon as any real anon figures it out. Shills like PM are useful for something.