The different methods are a byproduct of something called vblank. Vblank is an interrupt function that happens as a result of screen rendering and it executes an RNG call.
During wild encounters, the game picks a nature, then keeps rolling PIDs until it finds one that matches, then generates IVs. There is almost always a single vblank (dead RNG call) between battle start and the chosen PID. Sometimes two vblanks if enough PIDs are rerolled.
Other times that second vblank happens during the PID roll. Theoretically could be between the PID halves, between the PID and IVs, or the IVs themselves.
Method 1 is 0 or 1 vblank before PID is chosen.(often referred to as Method H1)
Method 2 is 1st or 2nd vblank between PID halves. (often referred to as Theoretical Method H3). This method is so exceedingly rare (nobody has reproduced one or found one in the wild) it is skipped where Method 3 is often called Method 2.
Method 3 is 1st or 2nd vblank between PID and IVs. (often referred to as Method H2)
Method 4 is 1st or 2nd vblank between IV halves. (Often referred to as Method H4)
The order is always the same (hence no CDAB). You can find it in PKHex's legality, including battle chance, first party check, nature confirmation, etc.