you can use 2N3055 with 7805,LM317 ,but the thing is there is a huge heat loss when using a linear regulator at 4.5A to reduce 12V to 5V.you can design a SMPS too.
check figure 5 for 7805 high current circuit
http://www.bristolwatch.com/ele/basic_power_supplies.htm
it's better to buy a buck...