[AUCTeX] input encoding, unicode, multi-byte with tex

[Ed C.]

I have a lot of characters (glyphs?)like:

LONGĒ  ANTEĀ quam sīve ego, sīve tū, nātī essēmus (Latin with macrons, 
in mule it is latin4) but I can't figure out how to make a pdf. Using:

\usepackage[latin4]{inputenc}

in the preamble generates a Latex error which I can't understand:

ERROR: Package inputenc Error: Keyboard character used is undefined

--- TeX said ---
(inputenc)                in inputencoding `latin4'.

See the inputenc package Hdocumentation for explanation.
Type  H <return>  for immediate help.

Capital H <ret> in the *Message buffer doesn't seem to do anything.

Thanks,

Ed