Some of you look at me in disbelief when I say you can put strands of yarn together to make up the different thicknesses so I thought I would try to explain it a little for you.
The names 4 ply - double knitting - chunky etc refer to the thickness of the yarn not the weight of the ball. The 'ply' term is an industry standard which refers to the thickness of a single strand of spun yarn.
4 ply is 4 tiny individual strands twisted together - double knitting is technically an 8 ply ( although because of the way they make it now you can't still split it into the 8 strands) - so if you use 2 strands of 4 ply together you will make up the double knitting thickness.
Chunky is twice the thickness of double knitting - so it you work 2 strands of double knitting together it is the same knitting thickness as a chunky. This means (and it took me ages to get my head round this one!) - a 100g ball of double knitting and a 100g ball of chunky is exactly the same thing! As the double knitting is half the weight - you get twice the length - but using 2 strands together it will work out the same as a chunky thickness. So if you see a chunky and a double in the same brand of the same yarn - and the chunky costs more - you will pay more for your yarn that you need to.
Super chunky is double the thickness of chunky - so if you use 2 strands of chunky together (or 4 strands of double) you can make the super chunky weight.
All you have to do is hold the strands together and make sure that they all go round the needle and through the loop.
I often mix chunky and double knitting together when I am making hats with motifs on - it's fine. If it's for a cushion, toy, bag or a hat it won't show - and it will use up your scraps.
Some wools are heavier in weight than acrylic and work and wash differently - I would not recommend you mix wool and acrylic on anything that is going to be work and washed regularly.
Also the dirt cheap imported yarns that you get from places like The Works can be thinner than you would expect and don't have the give of brands like my Robin - so just be aware of that.