The prepend
atom needs to be outside the quoted list, as the last argument to font-lock-add-keywords
:
(font-lock-add-keywords
'latex-mode
'(("\\(\\\\begin\\|\\\\end\\)\\(\{\\)\\(document\\)\\(\}\\)" (1 lawlist-face-a) (2 lawlist-face-b) (3 lawlist-face-c) (4 lawlist-face-d)))
'prepend)