as_4tuple

convert a Sequence's head into a 4-tuple.

ex.

[1,2,3,4,5].as_4tuple produces (1,2,3,4)
[1,2,3,4].as_4tuple produces (1,2,3,4)
[1,2,3].as_4tuple breaks the pre condition
[1,2].as_4tuple breaks the pre condition
[1].as_4tuple breaks the pre condition
[].as_4tuple breaks the pre condition