Master Lock No 4

The No. 4 is a pin-tumbler padlock made by Master Lock. The No. 4 uses a laminated brass padlock body with an internal key-in-knob-style cylinder. The inner cylinder uses four pin stacks and does not use any security pins.

The No. 4 is a common low security padlock used in the United States. It is one of a line of laminated brass padlocks that includes the Master Lock #2, #4, #6, and #8.

Principles of operation
The No. 4 cylinder is a pin-tumbler lock with four pin stacks and no security pins. The cylinder typically uses the M1 key profile but may use other profiles, as well. The standard No. 4 can be opened in both directions but the "commercial" series body restricts opening to clockwise rotation. The No. 4 is not a key retaining padlock.

Disassembly instructions
The No. 4 cannot be disassembled non-destructively. To disassemble it the rivets that hold the laminated body together must be removed and each layer removed individually. The inner cylinder itself can be disassembled in the same manner as a traditional pin-tumbler lock:

Notes
 * The No. 4 cylinder requires a small diameter plug follower.
 * The No. 4 plug is retained by a crimp, rather than a C-clip or cam. Above are the generic pin-tumbler disassembly instructions.

Vulnerabilities
The No. 4 is vulnerable to a wide range of attacks. The reduced pin tumbler count and poor manufacturing tolerances makes it an easy lock to pick. It's commonly given to locksport beginners as a "confidence lock"; a lock that is easy to open and inspires you to continue picking. The No. 4 may be vulnerable to one or more of the following:


 * Lockpicking
 * Key bumping
 * Impressioning
 * Decoding
 * Bypass
 * Destructive entry