Your son sounds a lot like my 9 year old at that age. He hated coloring, and still does. His handwriting still stinks, although he knew his letters before 1 1/2. If your son's just turning 4, maybe just don't push with the writing? There are other ways to work with letters - like the jumpstart games where you click on the correct letter to complete the word etc..

If he's a builder, get him some K-nex and ignore the ages on the box. You'd be amazed at the fine motor skills these kids can accomplish when they want to build something cool that really works (sticks, gears, motors .. much more fun than pencils:-) K-nex are one of those toys they can play with from when they are 3 until they are 11+ and stop using the instructions to begin inventing their own working machines.
On a positive note... my son with the terrible handwriting, whose K teacher complained that he never bothered to draw the pictures or color in the lines (she wanted him to draw 9 frogs, he would write 9 in binary code instead just to confuse her, or because he was bored, who knows?) .... anyway, he's doing fine in middle school and going to high school next year. He won't be an artist, that's for sure, but he may be a scientist who does all his illustrations in powerpoint:-)

Another suggestion: part of the reason my son hated writing, coloring, cutting etc.. was because he saw no purpose for these skills. "Why cut out shapes when it's hard for me and I could be learning something?" (Yes, learning to use scissors has a purpose, but cutting out random things seemed stupid to him!) He was ambidextrous until he chose a hand on the first day of K, so that added to the problem, but he also had no desire to just sit and write random letters for no reason. (He knew his letters, just saw no point in proving he could write them!) Maybe work on those skills as part of something he is interested in - for ex: Tell him you guys could creat a scientific journal of his bug hunts and record the bugs you find and help him write a list so he can keep track of what he's found and draw a little picture of what it looked like for future reference. He could cut out real pictures of bugs he's found or would like to find to put in his notebook. Maybe if he saw the purpose (for him) of doing these things in the context of something he's interested in, he would be more willing to give it a try?