Just a little bit list, more detail would be done on wiki soon.
1. NEO4 board size is 45 x 56mm, but M4 is 85 x 56mm
2. NEO4 has 1GB DDR3 RAM with single chanel, But M4 has two version 2GB DDR3 RAM/4G LPDDR3 RAM with Dual Chanel.
3. NEO4 will use AP6212 wireless module with single antenna , but M4 use AP6356S dual-band module, and use 2x2 MIMO and 2 real antennas.
4. NEO4 has one MIPI-CSI, M4 has two MIPI-CSI
5. NEO4 has USB3.0 x1 & USB 2.0 x1, but M4 has USB 3.0 x4 behind a VL817 internal hub.
6. NEO4 use 1.27mm pitch SMD connector for GPIO-40 pinout, M4 is same with RPi3 40pin GPIO.
Both have:
1. PCIe x2 pin-out
2. eMMC module connector
3. GigE port.
4. TypeC is for power supply and OTG.
5. HDMI-A & MicroSD slot.
6. Big CNC heat sink, with two side 1/4 screw hole