At this point U4 racing is kind of an afterthought for most. Many crawling events add a u4 event just for the fun of it and many people just use their crawlers to compete in it. Their really arn't any purpose built by manufactures kits availble for u4 racing. Bombers and bouncers are the most popular ones to use. The losi lasernut is probably the closest to ideal rig for U4. I would love to see this become more mainstream and become a thing. I have never competed in a u4 event I have only spectated. I race alot of offroad and do alot of comp style crawling so I would probably really enjoy U4.