Yesterday I reflowed a buck driver and tested it today :
6A, 1S to 1S, same 18mm clearance diameter and HDR of course, this one is 21.2mm for a H04.
It works very well and the efficiency is also very good for this small IC :
This is after I swapped the 0.9uH (8.8mΩ) inductor for a 0.47uH (4.6mΩ) one, but the efficiency was slightly higher with the 0.9uH at the two points I measured before swapping :
4Vin 1Aout : 97.2% vs 96.4%
4Vin 3Aout : 95.9% vs 95.5%
This is also with a quad SST-20 meaning a very low Vf and with a buck converter the higher the Voltage difference between the input and output the lower the efficiency.
The total resistance of the driver is ~37mΩ in dropout (100% duty cycle), not quite as low as a FET direct drive but still not too far from it.