Your mistake is in assuming there are two adjacent nouns. Rather, "four blocks" is, here, not a noun-phrase but part of the adjective-phrase "four blocks away". One could also more deeply analyse this as an adjectival use of an adverb ("away") by attaching it to the implied copula ("is", from the omitted "which is" which you already noted), modified by an adjunct noun-phrase ("four blocks").
