Four letters?
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Four characters.
The single prime, 4', does represent a length of four feet.
In computing, a block length is the length, in bytes or characters, of a block of data.
It means that any value that is stored in that part of the database must be exactly 10 characters in length - not more nor less.
A regular square or a rhombus if you mean all four sides are equal in length, a parallelogram if you mean the opposite sides are equal.