re: What size needles should I use?
the size suggested for your yarn, or 1 size smaller…
that may be a US7 (4.5mm) or a US 8 (5mm) or a US 9(5.5mm)
to some degree its up to YOU.
I like when making cables and ribs, to go smaller and would use a size 7 or 8 (medium weight yarn (#4) can be worsted or aran --i’d use 7 for worsted, 8 for aran.
the ball bands are likely to suggest 8 and 9–
but if you knit loose, you might want to go as small as 6 or 7…
(and if you knit tight, 8’s and 9’s!)
there is NO single answer.
it depend on your yarn, your gauge (even if there isn’t a gauge suggested, you get to decide what the fabric is light, soft and drapy and stretchy… or firm and detailed and less stretchy.
ME? i like firm and detailed and less stretchy…
but that’s ME and what i like!
YOU can agree… and go with smaller needles…
or you can disagree and go with bigger needles.
there is no RIGHT answer! Its YOUR knitting. Do what YOU like!