Using letters, numbers and symbols within a passcode offers the highest level of security, but it can get rather tiresome if you’re having to keep switching between keyboard layouts.
There is, however, a middle ground that offers all the convenience of a 4-digit PIN but with the benefit of increased security.
The following screencast is just one of many tips and tricks from my ebook The iOS Compendium, Volume 1 which is available on both the iBooks Store and this site in PDF format.