Sure, a simple passcode with four numbers will keep most casual folks out of your iPhone, but if you want it to be really secure, you should think about using an alphanumeric password, like you would on a website or your Mac.
The idea here is simple, the more characters you have (and the less obvious your password is), the better your security. Balancing a large enough number of characters with ease of recall can still be tricky, but I’d bet you’ve got it fairly worked out on the websites you visit — why not use that same acumen on your iOS devices?
Here’s how to turn off the simple passcode in iOS, and set up a more secure one.