Simplest/Cheapest Way to Create 4.5A Test-Load for 12Vdc Step-Down Regulator
Automotive incandescent ight bulbs are what most people use for 12V load testing.

These are available in power ratings starting at about 5 Watts all the way up 55 Watts or more. The ones that I use in my shop draw about 1A each and I simply use as many as required to get the desired load current. 

Incandescent lamps have the advantages of being low cost and readily available. 

These are 1141 lamps

Lamps similar to these and their sockets can be had almost for free if you visit a local automobile wrecking (salvage) yard.