Your son sounds very similar to my son. Do you have a homeschool community in your area? Going to park days regularly so he could form friendships with some of the other children really made a difference in getting him outside and engaging in active and imaginative play.
So my own philosophy is to avoid early academics. At 4, we did play-based "learning" and it was almost entirely child-led. We are mostly unschoolers, so we still are mostly child-led, but do use some curriculum-based programs for guidance and inspiration.
Remember that many high-achieving countries education systems avoid formal reading and writing instruction until around 7 years, as does Waldorf and other similar programs. There is a reason for this; most children's fine motor and visual skills simply aren't ready for reading and writing before then. If your son enjoys reading, great, take him to the library every week and let him pick out a ton of picture books. I wouldn't use "early readers" (boring!) or chapter books designed for young children; most picture books have higher vocabulary and better storylines.
Even if he's reading independently, his understanding level will be much higher than his reading level, and will continue to be so for many years. Reading aloud is so important at this age! Books such as "Some of My Best Friends are Books" by Judith Wynn Halsted and the "Read Aloud Handbook" by Jim Trelease will have many ideas for good books to read together. The 5 in a Row series will also have good ideas for books you can read together as well as activities surrounding the books which you can use as inspiration (personally, I felt that if we actually did everything as described it would remove a lot of joy from reading the books, but YMMV).
If he is really interested in more writing, then I liked the Handwriting without Tears series. There are workbooks and activities designed for pre-K and K; at 4, I'd use the pre-K program but again YMMV and you might want to pick up both for inspiration. The teachers guides are worth getting as well for ideas. As OPs have suggested, I'd do a lot of handcrafts and art at this age for fine motor practice and simply for fun! "Writing" in sand or with shaving cream, finger painting, chalk on the sidewalk, making letter shapes with sticks or rocks. . .Art supplies are one thing that I spend a fair amount of money on; I want my children to experience natural fibers, good-quality paints, etc. I like DIscount School Supply for cheaper items that they can use freely and excessively, but we try to also have some "real" supplies for use as well.
We really have liked the Explode the Code series for writing practice and reading/spelling rules. Yes, workbooks, but very short without needless repetition. We found them more helpful for our reluctant reader who wants more "rules" and less so for our child who seems to have picked up the rules of spelling through osmosis (keep reading aloud ;-).
Math - we use the RightStart math curriculum for one of my children. It's got very short lessons, an optional set of worksheets which we mostly avoid, and plenty of review (some of which we edit/skip to avoid boredom). Very game and manipulative based, which I like. We actually started it with her in K, but we started K a year later (when she was 5, almost 6). I also like the MathStart book series by Stuart Murphy, and use the livingmath website (http://livingmath.net/) for other book ideas - great website, BTW. Someone mentioned Life of Fred, which we use for our son who really intuitively understands math, but claims to "hate" it, and will just shut down immediately if I stick a worksheet in front of him ;-) Also like Kitchen Table Math and the Family Math series from the Lawrence Hall of Science (UC Berkeley). Family Math is game-based, lots of fun, quick simple ideas. I've heard good things about the Teaching Textbooks, which are computer-based, but are quite expensive. We also sometimes use the Youtube video series Khan Academy for specific math topics.
We sparingly use the Well-Trained Mind for guidelines on science and history. I was really turned off by her philosophy the first time I read it, she is really heavy into early academics and a very structured, intense, (IMO) rigid learning style. But I do like the division of science and history, the repetition so kids are introduced to topics at an increasingly higher level several times, and the non-Euro/Americo-centric nature of her history approach. I also appreciate that (at least the older editions of her book; I think the newer ones suggest the branded book and CD series now being marketed w/ the same name) rely heavily on library use, picture and literature books that are readily available.
I've already mentioned library use several times. We rely so heavily on our library, and are lucky to have access to two wonderfully rich library systems. We spend lots of time in the juvenile non-fiction sections, especially myths/legends/fables. Whenever one of my kids gets fascinated by something, I'll check out a few books on that topic, some that we might read together, others that I think they'll be interested in reading on their own. Again, I don't "assign" them, just put them out in an accessable place and let them enjoy.
Enjoy! It should be fun and non-stressful for you both at this age. I've found time and again that when my kids were "ready" both developmentally and interest-wise in learning something, it was quick and easy. I loved the early homeschool years because I was learning and playing as much as they were. Kids are naturally curious and exploring at this age - take advantage of it!