Apparently the power supply will draw about 4.5 amps. A car battery puts out hundreds of amps, but a common misconception is that those figures have to match. That is not the case, the power supply will ONLY draw the amperage that it needs from the battery. it could be hooked up to a 10,000 amp battery, and it would still only use 4.5 amps. The only worry would be the fact that a car's 12v is not true 12 volts, it varies anywhere from 8 to 14+ volts, which can damage a power supply designed for exactly 12 volts. That is why Hijinks recommended a voltage regulator, to keep the voltage at 12 volts.
Most likely hooking it up to your car battery will not hurt it, as long as the engine isn't running. Do a search of these forums for 12v regulator or similar and you will find out all kinds of info,,
My 2 cents