What US Needle Size Is 4mm?
US 6. A 4mm needle is UK/Canada old-size 8 as well — three names for the identical physical tool, since metric is the one sizing system that describes the needle's actual diameter directly rather than an assigned index number.
Why metric is the anchor, not US or UK
US and UK/Canada sizing are both arbitrary index numbers assigned to a needle diameter — neither one describes the actual measurement the way millimetres do. This is exactly why a metric size never needs converting to know what it means physically, while a US or UK number always needs to be checked against a chart (or run through the Needle & Hook Size Converter) before you know the real diameter in hand.
Where 4mm sits in the wider table
Between US 5 (3.75mm, UK 9) and US 7 (4.5mm, UK 7), US 6 is squarely a mid-range needle, commonly the go-to size for worsted-weight yarn's lower gauge range and a frequent choice for a firmer DK-weight fabric.
The UK number runs backward from the US one
Notice that UK 8 is actually the finer needle of this pair, not the bigger one — a US 6/UK 8 needle is thinner than a US 7/UK 7 needle, even though 8 outranks 7 numerically. The how-to-convert-UK-to-US answer walks through why UK counts this way in full.
FAQ
Is there a crochet hook that matches a US 6 / 4mm knitting needle? Yes — a G-6 crochet hook is also 4mm, making it the closest crochet-side match by diameter, though hook and needle sizing don't always translate directly to matching fabric density for a given yarn.
Does 4mm always mean the identical needle regardless of brand? Yes for the metric measurement itself — a 4mm needle is 4mm across regardless of manufacturer; what can vary between brands is the tip shape and material, not the diameter a metric size specifies.
What yarn weight is 4mm typically used with? Worsted weight most commonly, sitting toward the finer end of worsted's usual needle range — DK-weight yarn also sometimes uses 4mm for a firmer, denser fabric than DK's own typical needle size would produce.
Is a 4mm circular needle sized differently from a 4mm straight needle? No — the metric size describes the needle's shaft diameter regardless of shape, so a 4mm circular and a 4mm straight needle produce identical gauge in the identical yarn; the choice between them is about construction, not size.