Given a list of lowercase strings, find the length of the longest possible chain of words where each word in the chain is formed by adding exactly one character to the previous word (the relative order of existing characters must be preserved). A word can appear multiple times in input but should be treated as the same word for chaining. Return the length of the longest chain (a single word counts as length 1).
['a', 'b', 'ba', 'bca', 'bda', 'bdca']4