The four sentences (labelled 1, 2, 3, and 4) given in this question, when…

Concept. A jumbled paragraph is reassembled from two objective constraints, not from what merely sounds fluent. Referential dependency: a pronoun or a definite phrase can only appear after the thing it points to has already been named, so the opening sentence is the one that introduces its subject with no unresolved back-reference. Chronological progression: narrative sentences advance in time, and markers such as "eventual", "after treatment" and "Then" fix where a sentence sits on that timeline.

Application. Apply the two constraints to the four given sentences.

1. Sentence 4 is the one sentence that names its subject outright — the lump on Cathy Perkins’ index finger — and introduces it for the first time. Every other sentence carries a reference that would be left dangling at the start, so 4 must open the paragraph.

2. Sentence 2 says the viola player "didn’t know what it was". Both "the viola player" (Cathy) and "what it was" (the lump) need sentence 4 before them, so 2 comes immediately after 4.

3. Sentence 1 resolves the uncertainty raised in 2: "The eventual diagnosis was skin cancer". The word "eventual" presupposes the preceding period of not knowing, and "after treatment all seemed well" moves the timeline forward, so 1 follows 2.

4. Sentence 3 opens with "Then" and reports that "it had come back and spread to her lungs". A recurrence is only possible once the disease has been named and treated in sentence 1, so 3 closes the paragraph.

Cross-check. Test the assembled order 4 → 2 → 1 → 3 from both directions.

• Reference test: the lump is introduced (4) → "what it was" now has an antecedent (2) → "the eventual diagnosis" answers it (1) → "it had come back" points to the named cancer (3). Every reference looks backwards at something already stated.

• Timeline test: a lump appears → nobody can identify it → it is diagnosed and treated → a scan later shows recurrence and spread. The events run strictly forward in time with no jumps.

• Any arrangement that begins with 1, 2 or 3 opens on a dangling reference — "the eventual diagnosis" of what, "what it was" about what, "it had come back" after what — so none of those can be the opening sentence.

Therefore the coherent paragraph runs 4 → 2 → 1 → 3, keyed in as 4213.