How do I convert a pattern that asks for 4 ply when I have\want to use dk. Do I use a smaller crochet hook?

You could use a smaller hook but the knitting would be very stiff and horrible.

Changing the yarn that much needs the pattern to be rewritten not tweaked. It's simple for someone with basic maths but most people are scared to try.

www.knittingpatterncentral.com has lots of free patterns. You will probably find something similar in your yarn

If you want your item to turn out like the pattern you've chosen, you're going to have to buy lighter yarn. If the pattern is for something that doesn't have to fit, like amigurumi or a shawl, go ahead and make it in your heavier yarn, using a crochet hook appropriate for the heavier yarn. Just be aware that the resulting item is going to be larger than the item in your pattern.