A 4-foot magnet-mount antenna on the roof of a vehicle with a 4-watt hand-held does as well as most other legal CB radios out there, with a range of maybe five miles or so in good conditions. That's basically what I have on my sedan for use on long highway trips (although not using a hand-held), listening for and giving traffic info when needed, e.g. traffic holdups, wrecks, etc that warrant consideration of immediate rerouting.
The problem comes when the antenna is attached just to the hand-held radio: it's pretty unwieldy to carry a 4-foot antenna, and the ground plane is terrible, resulting in a useful range probably a mile or less. It would probably range better than a 27-inch antenna, but still not good for most purposes.