Right gauge, wrong yarn?

Hi,
I have a pattern for a sweater I love ( It is on the cover of Hollywood Knits Style by Suss Cousins, the one with the zippers)
It is in Suss Bomull a 4ply cotton, gauge: 16 stitches = 4 " on 6mm needles,
My problem is I can’t get that yarn in Australia, I have found a 5ply acrylic cotton mix and I have done a swatch and it came out the right size. But it looks loose and open weave surely this can’t be a wrong subsitute.

PLEASE HELP !!

Here’s what I’m thinking may be the issue: I went on Amazon and took a look at the cover of the book as an enlarged image. It does look like a fairly tight fabric. At 4 sts per inch on 6mm needles, that means it’s a fairly substantial yarn, almost an Aran weight. So what may be going on is that “ply” is different in the US. Worsted weight is often refered to as 4-ply in America, where as that is a very thin yarn in the UK, and I would imagine maybe also in Oz. Can you find a cotton or cotton mix which is closer to a worsted weight or Aran weight?

You’d want a thicker yarn; the guage for that yarn - http://sussdesigns.com/knittingSite/productDetail.php?prodID=28 - would put it at about 8 ply in your system. Or maybe 10 - we’d call it worsted weight. The 4 ply referred to in the pattern isn’t the same as your 4 ply.

OOPS didn’t know about the ply thing, Would it work out if I used two strands of this yarn together and try out different needle sizes?
The yarn I was going to use is Sullivans Cotone but it doesn’t say how many yards per ball?

It may work out doubled, you’ll just have to try it out and see how it looks.

I’m in Australia too and would suggest using Bendigo’s 10 ply cotton. A great Aussie product at a good price
http://www.bendigowoollenmills.com.au/