I don't know that it's typical of kids who like math. however, it may be indicative of your son's learning style. Or your son sounds like he might fall into the gifted category.
If you aren't familiar with the ACTUAL definition, "gifted" is somewhat of a hot button with mamas and educators, as it can be misdefined or mislabeled. Many people who don't know much about "gifted" kids will use the term to mean they think their kid is a genius...lol. HOWEVER, truly 'gifted' kids are typically IMBALANCED learners. Which means they master one skill above their "age" but may lag behind in others. it will all even out by college....lol. However, in the younger years, especially, it means you can have a kid who is doing 1st or 2nd grade math, but is still pre-kindergarten in reading or writing.
At 4 it's really hard to tell. Although some schools have gifted programs beginning in Kindergarten and so they will offer testing this young. Typically, you would notice this and test for it in 1st or 2nd grade. You can do some research online and see if your son has more charachteristics of a gifted child.
He also may just "seem" behind in writing, because he is advanced in math.... therefore the filter is off. So the answer is just to keep him excited about math. Most of the time kids who are good at math will excel with a fretted instrument (guitar, violin) or the piano. So, this would also be a good avenue to see if he has interest and get him some lessons.
Make sure he is working out his hands to build nimble gross motor function ability, if he doesn't want to build these muscles with writing. Play with playdough, lace shoes, button shirts, put together legos and models with small pieces. That way you know it's not a gross motor function issue (ie - it's physically hard for him to write rather than a processing challenge or stubborness issue). You can also get color by number sheets that have abstract designs rather than pictures. Then you could say - color red a total of shapes that would add up to 12. (so he would color one area that has a 1, one area that has a 3, one area that has a 4 and two areas that have a 2 - OR two areas that have a 3, one area that has a 4 and one area that has a 2, which these choices would increase his ability to solve problems from different angles.) Then, color blue a total of shapes that add up to 4 etc etc etc.
The MOST important thing is to do what you are doing..... get him the tools to use to go at their own pace and don't push when they seem to be over it. But keep going back and revisiting. He will have to eventually build himself a toolbox so that he can write at an acceptable level to hand in documents etc. Sometimes typing is the answer, but there are many other tools if he continues to struggle.
One of the things I would do is take my daughter to the store and have her help me figure out the best deals - soup is easy because it's usually even price points.... should we get the 3 for $2 brand or the 4 for $5 brand. Help him break it down and then figure out which is the best deal. Even if he CAN'T do that yet, this is a great critical thinking exercise to hear YOU do outloud (even though I always had to use a calculator....lol)
Good luck