Usually got sticker on the gearbox.. if not, check for markings.
The Y21 and the S4C are both 4.4, but the final have different mounting holes diameter for the LSD. S4C can use EK9 LSD, Y21 cannot.
S80 also has two versions.. one with the 4.4, ealier gen, and then the later gen which is S80, but come with 4.785FD.
If you're using the gearbox from EK9, most likely most aftermarket Japan LSD will fit, but check first.. if you get extremely lucky, sometimes you can open up the gearbox and find that it's already modified with a different final drive. haha.
If you're using Y21, tough luck.. you have to use the EG LSD, as far as I know.