If your Mobo has got enough room for 4 case fans,there's no better choice since the board monitors the fans according to your temperatures. But if you're left with no fan headers,then the PSU is your only choice. Though,if you connect the fans directly to the PSU,what you should keep in mind is that your fans will be rotating at full speed,with no fan speed control,which may end up in your PC becoming more noisy. Though,don't worry your case will stay cool.
So in short,go with the motherboard,if you have the required fan headers,and go with the PSU connection if you're left with no other choice.