Having some kind of lock on your smartphone is an essential part of your data security. While any kind of lock is better than none at all, some methods of locking your phone are more secure than others.
Using a long passcode offers you the best security. A 4-digit PIN is easy to quickly enter on your phone, but it is also easier to crack than one just a couple of numbers longer. For example, if you use a 6-digit PIN it adds an additional 990,000 additional combinations that must be tried to be sure of a successful brute force attack. A PIN of 10-digits or more pretty much ensures that you are protected from a brute force attack (at least based on today’s technology). Android lets you go up to 16 characters (under Screen lock in Security in Settings), while on iOS you get up to 37 characters (tap Touch ID & Passcode or Face ID & Passcode from Settings).
Avoid biometrics (like fingerprint or facial recognition) as these can be spoofed, and you have no real protection if you are compelled to provide your fingerprints or photo (face scan). Swipe patterns are also pretty weak, since they can often be guessed, and if someone sees you enter the swipe pattern or can determine it from smudges on your phone’s screen the pattern can be cracked in just a few guesses.
So, while your thinking about it, upgrade the PIN on your phone to at least 10 characters that you can easily remember.