I think you are maybe mixing up a couple of things-- There are plys and there are weights. They used to have much more to do with each other than they do now. Older yarns in the UK and the Empire (Australia, Canada, etc.) were listed in plys. Now, weights are used. And the term "weight" is deceptive (don't worry-- it does get simpler and I'm going to get to the truly helpful part in a minute:) )-- it really means thickness. The way the yarn is spun and the material used can make a 100g ball of yarn give you 80 yards or 100 yards and still have the same thickness and gauge. There are 7 standard weights, 0 - 6, and they really mean thickness. When you look on a ball of commercial yarn, on the band, there will be a little drawing of a skein and a number on it, telling you which weight. DK weight is a 3. A lot of worsted weight, which is a 4, is 4-ply, and I think that's what you might be talking about.
The gauge is what really matters. If the gauge says something like 6 sts per inch, then that's what you need to get to make the clothing come out right. Whatever needle size works for you to get that gauge, is what's important. The needle size they recommend (3.25mm and 4mm) are only the sizes the pattern writer and pattern testers used to get the right gauge. That doesn't mean that those are the sizes that will be right for you. Use whatever needle size works for you to get the gauge and you'll be all set. For a swatch, just cast on 10 sts and knit for an inch or 2 and measure off an inch across. Let's pretend the pattern says 6 sts per inch. Count how many sts you have and if it's 6, then that's the right needle size. If you get too many sts, say 7 or 8, then use a bigger needle and try again. If you have too few, say 4 or 5, then use a smaller needle.
If the pattern calls for DK weight, then I would get yarn that has a 3 on the band and try different needle sizes until you get to the gauge in the pattern.