You check for the length:
len(x) >= 4
or you catch the IndexError
exception:
try:
value = x[3]
except IndexError:
value = None # no 4th index
What you use depends on how often you can expect there to be a 4th value. If it is usually there, use the exception handler (better to ask forgiveness); if you mostly do not have a 4th value, test for the length (look before you leap).