> 1) Lisp/Scheme
> 2) Erlang
> 3) Smalltalk
> 4) C
>

Personally I think Smalltalk is not that interesting any longer. Many
of its ideas are found in other languages, including Ruby. So if you
want to learn a new language for the experience, I wouldn't go with
Smalltalk.

C is a pain, and I try to avoid as much as I can. If you have limited
time, C would not be a good choice... you will spend time compiling
and recompiling to fix some stupid pointer error... grrr... That said
if you want CV skills, C is definitely very important.

Lisp/Scheme and Erlang seem both pretty interesting. I've been meaning
to try something from the Lisp family for some time but so far I have
postponed the effort.

The one I am considering getting into and that may interest you is
Scala. It is useful in that it's very close to Java so you get good
compilers and a strong community, plus you could use it for many jobs.
It takes several ideas from Ruby (I find the syntax similar) which I
find nice, but you can also have things like Actors (a la Erlang) via
its powerful libraries.

So personally I'd suggest:
- Lisp/Scheme for the experience
- Scala: a bit easier to approach and more useful while still exposing
you to new ideas and worlds.

Hope that helps.

Diego