while si-adjectives should be noted in singular,
any adjective that counts the shades of a specific color should be noted in plural.
I think that “4-grays LCD” is not standard English.
With Google, I got 32 results for “4-grays LCD” and 1560 results for “4-gray LCD”. Example: “Transforming the crisp b/w LCD display into a flashing 4-gray LCD” (Software for the OnHandPc : Games, Clocks, and Applications).
“4-gray LCD” has the same structure as terms such as “3-lever lock”. The term “3-levers lock” is not standard English.
A. I’ve made the same search, but most of the results I got for “4-gray LCD” were the result of context altering rephrasing by Google.
EG1: 20x4, gray LCD
EG2: 4", gray LCD
EG3: Epoc 4, gray LCD
B. “3-lever lock” is an all-instances count (there are always 3 levers on the lock), “4-grays” is a possible-instances count (it’s not a given for all four grays to be shown at any given moment).
C. “gray”/“gy” is the si-equivalent to the “rad” (1 rad =0.01gy). using “4-grays” makes it clear that we are talking about color-depth and not about radiation level. (nor, on a lesser note, a “4-bit Gray code”.)
Despite your reasoning in B and C, ‘4-grays’ is not standard English. Refer to Chicago Manual of Style, 15th edition, section 5.92 (the ‘{’ and ‘}’ brackets are in the original):
A phrasal adjective (also called a compound modifier) is a phrase that functions as a unit to modify a noun… (5) If the phrasal adjective denotes an amount or a duration, plurals should be dropped. For instance, pregnancy lasts nine months but is a nine-month pregnancy, and a shop open twenty-four hours a day requires a twenty-four-hour-a-day schedule. The plural is retained only for fractions {a two-thirds majority}.