One word, two instruments. The Puerto Rican cuatro is ten steel strings in five doubled courses, played with a pick. The Venezuelan cuatro is four nylon strings on a small box, strummed with a stroke that damps as much as it sounds. The comparison page covers why one word ended up on both; these pages cover what that means when you are buying, stringing or adjusting one.
They are explainers, not a catalog. No prices, no brands, no reviews — this site has not handled the instrument you are considering. What it can do is tell you which instrument you are actually looking at, what to check on it, and what each adjustment changes.
Where to start
If you are not sure which one you want, start with your first cuatro — the tell is the number of tuners. If you own one and it sounds tired, strings is the page, and the symptoms differ between the two instruments. If it sounds right and fights you, go to setup.