To turn a heel I have to follow a pattern, so I would use 4 needles for that if the pattern said so. This is because it will state which stitches are on which needle and all instructions would be for that situation. But for the leg and the straight part of the foot I would use a 5 needle set, because it's easier. With 4 needles you are knitting in a triangle, which can be quite tight, but with 5 you are knitting in a square (plus the fifth to knit onto) and I find this easier. You could probably use any number
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I now have the details of the sock book I recommended: The Twisted Sisters Sock Workbook by Lynne Vogel, Interweave Press. Mine was £11.50 but if I'd searched around a bit I'm sure I could have got it for less. It contains a basic sock pattern and then lots of variations for the heel, and even how to knit a sock in the flat on 2 needles (then add in the heel) The book teaches lots of the tricks of the trade and makes it all seem simple. There is a large section on spinning and dyeing for socks, which many would not be interested in, but overall it provides just the information and inspiration needed to get down to it.
Have a look at
www.interweave.com there are loads of books and patterns, although none of the patterns is free
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I do have another sock book which was very helpful - must try to find it.