The IQaudIO DAC+ should definitely work with the Pi 4. The Pi 4 is especially recommended for use as a server, as it is equipped with more RAM than the Pi 3 (at least the versions over 1GB). Apart from that, the Pi 4 can be set up with Max2Play just like the Pi 3 or older versions. All it needs is the newer buster image, which is currently in beta. Nonetheless, all features should work normally and we try to fix any occurring bugs as soon as possible.