One being spiteful about letter going too far (1,3,4)
I believe the answer is:
a bit much
'going too far' is the definition.
'one being spiteful about letter' is the wordplay.
'one being spiteful' becomes 'a bitch'.
'about' means one lot of letters goes inside another.
'letter' becomes 'mu' (mu is a kind of letter).
'abitch' enclosing 'mu' is 'A BIT MUCH'.
(Other definitions for a bit much that I've seen before include "unacceptable" , "out of order" , "Contradiction in terms" , "over the top" , "Too much to put up with, unreasonable" .)