Never leave your mobile phone unattended, without a security password in place. Activate your smartphone to auto lock the screen after a short period of inactivity, like 15 seconds.
4 digit PINs are the easiest to break, so you should skip using those and instead set a good password, similar to those you use for your online accounts. That means it’s long, random, with mixed lower and upper cases, digits and symbols.
Or draw a pattern.
Or, even better, activate fingerprint authentication, if that’s available on your device. It won’t be a secret, as we leave our fingerprints everywhere, but biometrics are the hardest to replicate.