Knitting a Cardigan from a Pattern Using Different Yarn

Hi everyone, can anyone help me?
I was super motivated to start a new project, but I realized I’m missing some key knowledge.
I bought a book with patterns for cardigans.
But I made a mistake when buying the yarn. Instead of size 8 yarn, as specified in the pattern, I bought size 3–4 yarn.
Is there a way to adjust the pattern, or do I need to buy a different kind of yarn?

Can’t you just use two strands at once? You’ll need to do a gauge swatch, of course, but it should come out pretty close.

Hey there.... 
Actually, you don’t need to buy new yarn. Although I’d find that option very tempting 😄 
You’ll need to check what the gauge says on the yarn label and compare it to the one in the pattern. Then, in theory, you could recalculate everything. Sometimes you get lucky and it’s enough to just knit the project one or two sizes larger. Depending on the project, though, that luck might be misleading. Because the proportions might not quite be right anymore (armholes, etc.).... 
Bottom line: buy new yarn or choose a different project. 

If the pattern gives measurements in centimeters, you can do a gauge swatch and use the rule of three to convert the required stitches and rows. (You wouldn’t believe what math can be good for sometimes 🙂)

Yes, the measurements are listed for each size. 
Thanks for your quick reply.