Are you expecting to get 60 Watts (5Amps at 12Volts) of power from your 4-20mA control signal? If so, there just isn't enough power to start with.
4-20mA control signals seldom have more than 24 volts behind them, and often much less. So, the most power that could be extracted from them would be less than 0.5 watts.
Now, if you have another source of power to supply the 60 watts, but you need to control it from the 4-20mA signal, then the link that johansen posted, plus a power amp, might do the trick.