I use a DigiOne Signature fitted to Pi 3b+ running the Moode software, using BubbleUPnp on my Android phone or tablet. As the Signature requires 2 power supplies (at least one of them preferably being a 'clean' supply), I use a Temple Audio Supercharger which has enough power to power both the Pi and the Signature board, using a Y cable supplied by Temple Audio.
I also tried the Signature board fitted to a Pi4b but prefer the sound from the 3b+, which has a slightly less powerful processor than the Pi4. Apparently, a lower processed speed produces less noise.
I do intend to try the Pi4 again though, to be 100% sure, although I have compared them twice previously. There are settings in Moode (and LMS, actually) that can be tweaked too, that improve the sound - input and output buffer/cache sizes and Cpu governor (processor speed).
The pi/Digione Signature is a great streamer and easily bested the Squeezebox Touch that I had previously. The Digione Signature is audibly better than the standard Digione that I tried too. The Temple Audio Supercharger is a worthwhile improvement over the standard Pi power supplies.
As my DAC doesn't have USB input, I haven't been able to try the Pi 4b USB output into the DAC but there are a few on here that say it works really well.