Don't wrap lines with a four-space prefix (or whatever format is decided), wrap lines without one. Allow users to disable the wrapping if they prefer wrapped code. The prefix can be stripped at display time if you wish, so that code is left-aligned - actually, since the formatting isn't encoded in the log, only the semantics, users can configure the display as they please.

Possible handle lines beginning with - as lists and indent them correctly. If a message somehow breaks the format, do not accept it. This isn't TeX-level complexity.