I was wondering 4ply too, is it going to be a very fine knit?
Quality of wool can count too as cheap and nasty can be a pain to knit with and bring the quality down. I'd check any ideas for wool past the knitter first.
Is there a Hobbycraft near you? They do quite a selection of wool.
http://direct.hobbycraft.co.uk/shop/knitting_yarn
I like to get it in person rather than order online so I can feel it, check the quality, texture etc...
Whereabouts are you? People might know more local places you could ask in, smaller places more likely to have knowledgeable people who can help though John Lewis, albeit pricy have a great habdashery department and may have someone there who can guide you, especially if you take a pattern or guide (as to amount of wool needed etc...).