14 DAY TRIAL // With the iOS 9 release, Apple requires that all users will have to use the six-digit passcodes on all Touch ID-enabled iOS devices, which should make it a lot harder for anyone who does not know the actual passcode to access your data.
As IBM explains in one of their infographics, the old four-digit passcode was quite easy to crack even though, to do that, one would have to try 10,000 different combinations and, with the proper tools, this could be achieved in about 18 minutes.
On the other hand, the new six-digit alphanumeric passcode takes 1,000,000 combinations to hack, this amounting to 196 years if a cybercriminal used the same tools to crack the four-digit passcode in 18 minutes.
The six-digit passcode coupled with Apple's two-factor authentication added in iOS 9 should make it a lot harder for third parties to get access to your data.
The contrast between four- and six-digit passcode and what the two types of passcodes mean for mobile security is also presented on IBM's Security Intelligence blog.
Just in case you want any help visualizing the difference between Apple's old four-digit passcode and the new six-digit one, I appended IBM's infographic describing this subject below.
Update: Corrected the first paragraph to say that the six-digit passcodes are the minimum requirement for iOS devices with TouchID.