gonna build a 16 inch bike for my 4yr old.are parts for these different then full size as far as sprocket,cranks ,headset,etc....?

Don't know anything about it. But like you I am building a bike for a 3 yr old. It will be an 18" complete ground up build smaller than a micro mini! Good luck on your build.

thanks,i did find out that everything is the same except you need a 39 tooth or smaller sprocket and cranks should usually be around 155 mm.

Is he a big or small 4 yr old? FYI my guess is the 155 cranks will be way to big for him. My eight yr old has 155's on his Supercross and he is average height. My guess is 135 would be closer to what he would need. Now that I think about it...39 tooth is all dependant on what the rear wheel is. My eight year old runs a 37/15 I think.

actually its for my daughter and she is about average for a 4yr old.i have a set of 6 inch cranks off another bike i may throw on there.i just bought a used 39 tooth off another site that i will have powder coated.