BJT's (particularly Darlingtons like the TIP120/121/122) are not the ideal choice for switching applications such as yours.

Looking at the datasheet, the CE saturation voltage of a TIP12x is about 3V at a collector current of 4A. So 1/4 of your lights will give 12W (3x4) dissipation in the TIP. Max collector current of the TIP is just 5A. You won't be able to get the 15A you need out of them.

Using a MOSFET with a RDS on of 0.010 Ohm (such as an FDD10AN06A0) the power dissipated will 0.16W (4x4*0.1) at 4A current. So the TIP (12/0.16) 75 times power as the MOSFET - big difference. Also, the FDD has a max. current rating of 50A.

You can switch the FDD at several hundred KHz.