So I assume this one, or similar, would be more than sufficient?
But how do I connect the pis to it? Just connect all four in parallel to the output terminals?
Yes, connect them in parallel... GND & 5V to each RPi. Don't daisy-chain them -- in that case to carry 12A the wires from the PSU to the first RPI would need to be 11AWG (2.3mm diameter conductor!) (https://www.powerstream.com/Wire_Size.htm). For 3A, you only need 1mm dia conductors. At low voltages, shorter wires (within reason) are better.
NB: Cheap USB cables have less than 1mm dia wires