I have a yarn weight question

What happens if you use a 4 weight instead of a 3 which the pattern calls for? Will it make that much of a difference?

A 4 is thicker than a 3, so if you knit it on the same size needles as a 3 it will be much stiffer and larger. If you use larger needles it’s going to come out very much larger. You can follow a smaller size of the pattern or use less stitches.