The 3B+ has a Gigabit Ethernet, While it is a little limited is sufficient. The 1GB of memory is also just fine. The 4B+ is a true Gigabit implementation, it is not crippled by the USB interface, and you can get up to 4GB of ram. But as Michael said, you may need to have a fan to deal with heat in a small case......and likely you are going to be limited by the NAS anyway.

Where the 4B is a benefit it you have a USB3 SSD with your music on.....