Can't manage to sort a file and display time & date on a LCD screen.

You could recover some by using the F() macro:
Serial.println(F("initialization failed!"));

That won't be enough, the programme is not completely written. I should switch for a mega, shouldn't I?

4, just like you had.

So what were you trying me to do? If it is not that, what's the matter with pin 4? I am bad at Arduino programming.