Typically, when someone says 2x4, they're referring to the 8x2x4 piece of wood that you'd find as studs in your walls, at a lumber yard, Home Depot, Lowes, etc. However, as ChrisSlicks has said, they're actually a .5" smaller than what they're listed as.
Anyway, with lower cars, sometimes you can't get them up ramps or get a floor jack underneath. To make it possible, you set a 2x4 or 2x6 or 2x8...and so on. The wider the better....makes it easier to stay on top of the wood. With a 2x4, it's hard to rest the wheels on top without sever tries because the surface area is narrow. I use a 2x10. So anyway, you set whatever piece of wood you like in front of both wheels and rest the wheels on top of it. With that, you can slide a jack underneath with more clearance or set a set of ramps in front of the wood and drive up them.
Hope that helps.
