RPI4 has better I/O capability and can manage the Ethernet and USB data ports simultaneously at any audio data rate without clicks, pops or interruption. So it’s the best solution for a USB dac if you don’t want to buy a HAT USB bridge for a RPi3 - which would end up costing more anyway.
RPi3 can manage the Ethernet port and address a HAT DAC, or a HAT USB bridge without interruption because it does not have any contention with the (unused) USB ports.
RPi3 has less digital activity than the more compute capable RPi4 so might generate a little less USB ‘noise’. So potentially lower noise levels injected into the DAC.
There may be some merit in using a better power supply if you’ve got a HAT DAC. People often use a switch mode power supply from IFI (sensible). Some people even buy expensive linear power supplies and end up spending more on the power supply than they might on a decent USB dac with its own built in power!
Having said that I run a RPi4 with the standard power brick and a stock USB cable into a Topping D90 USB DAC and cannot hear anything other than background recording studio noise on a near silent section of a 24/96 recording. Nothing on a ‘scope either.