My oldest son will be 4 at the end of January and also is very resistant to practicing handwriting. However, he does seem more interested when it involves a topic he likes (as another respondent mentioned), for example Bob the Builder. If you are not opposed to bribes, you could also try offering a small prize or treat (I used M&Ms) for successfully completed rows of letters. I also tried saying things like "On this page you are supposed to write an entire row of zeros on your own. It's really hard to write the numbers on your own without tracing the dotted line. You probably can't do it, but if you can, I'll give you an M&M." This kind of reverse psychology works well with my son, who loves to prove he can do things I didn't expect, but I'm sure that how well this works depends on a kid's personality.
Ultimately, I thik kids will show interest when they're ready, and I try not to push things. If your son doesn't want to do this now, focus on the things he DOES want to do and is good at, and work on those.