This above a wd my passport at around same price?
The only way to fit a HD larger than 2TB inside a PS4 is either to buy SSD or you modify the chassis. There are no 4TB HDD that can naturally fit inside a PS4 as they are too thick. Modifying the chassis will immediately invalidate any warranty on your PS4. As you can just plug this device into your PS4 and use it that way, really makes no sense damaging your console just to use this internally.
Yes, you can ude the internal SATA drive. The drive inside is a (thick) Seagate Barracuda with a very small USB to SATA adapter board. One thing to be aware of is that the M3 comes with 3 years warranty, whereas the internal drive is not recognised by Seagate to be under any warranty. You will destroy the case when removing the drive, so if you want the internal drive you very likely forfeit any warranty.
Can this be disassembled to use inside a ps4? I am aware that it is most likely a thicker than the original ps4 drive, but there are some YouTube videos that show that the thicker Sat 2.5" drive can be wedged into ps4.