US 4 Knitting Needle: Metric & UK Size
US 4 measures 3.5mm, and has no standard published UK/Canada (old) equivalent — one of a handful of US/metric sizes that falls between two UK old numbers rather than lining up with one directly.
What it's typically used for
Squarely inside sport weight's standard recommended range (US 3-5), US 4 is a common choice for sport-weight cardigans, baby garments, and lightweight accessories where a mid-range gauge — not the tightest or loosest option — is wanted. It's also occasionally used for a firmer-than-standard DK-weight fabric, since it sits just below DK's own recommended range (US 5-7).
US 3 and US 5 as reference points
US 3 (3.25mm, UK 10) is the next size finer; US 5 (3.75mm, UK 9) the next size up. A knitter working from a vintage UK pattern that seems to call for something between UK 10 and UK 9 has genuinely landed on a size the old system never separately numbered — 3.5mm simply wasn't assigned its own UK slot, not that a UK equivalent exists but is obscure.
Gauge implications
Sport weight's published range (23-26 stitches per 4 inches) tends to sit mid-way around a US 4 needle for most sport yarns — neither the tightest nor loosest option within sport's recommended tool range. Swatching US 3, US 4, and US 5 side by side is a fast way to see the real gauge spread a given sport-weight yarn produces before committing to a size, an approach covered more fully in adjusting needle size for gauge.
FAQ
Why doesn't US 4 have a UK old equivalent when most other sizes do? The historical UK old sizing system was based on a physical steel wire gauge with its own fixed set of hole sizes, and 3.5mm simply didn't correspond to one of those holes — not every metric size in between the officially gauged points received its own UK number.
Is US 4 close enough to US 3 or US 5 to substitute freely? Not for a precisely gauge-matched project — even a quarter-millimetre difference adds up once it's multiplied across every stitch in a full piece. Swatch at the specific size the pattern calls for before assuming a neighboring size is close enough.
What crochet hook is closest in diameter to a US 4 needle? An E-4 hook (3.5mm) matches exactly, though as with any cross-craft size comparison, matching diameters doesn't mean matching gauge or fabric density.