On the other hand if you use a more complex pass code with more than four digits not only did they not know how many digits you using but you're more likely to repeat a digit. So it makes it much more complicated.
On the iPhone you can change your pass code to be more complex by using the settings application. The exact option is different if you have an iPhone 5S as opposed to an earlier iPhone, but it's in the settings application. On the 5S it is under the touch options on the earlier versions it's under the general options. In any case look for it and you can choose a five or six or his many digit pass code as you like and that makes it a lot hard both for someone to guess the code for someone to look at your smudgy phone and know what keys you use. Not the fingers are dirty or anything.