HP ProDesk 400 G4 SFF Hardware Reference Manual page 36

If you are removing a 2.5-inch hard drive, remove the four screws that secure the drive to the drive
●
cage (1) to release the drive (2).
11. Install the new drive in the drive cage.
If you are installing a 3.5-inch hard drive, slide the drive into the drive cage (1) and secure the drive
●
with four 6-32 standard screws.
NOTE:
cage.
28 Chapter 2 Hardware upgrades
The four screw holes for the 3.5-inch hard drive are stamped "A" on the top of the drive