As one of developers confirmed that: Anyone has found it already?

Nope, but i'd find that kinda interesting as well.

(You have to select 4 characters for it to even let you start the game... so somewhere there has to be started.)

Or maybe he just means that you can let your other 3 dudes die. But that is probably not it, since they revive at each crystal.

Maybe a future feature o.o;;

No, it is another way.He said "there is", not "there will be".

How about...since party members don't auto-attack, you can just have one chr. do all the attacking and not use/ignore the others? ;)

It is simply done BUT the others are still "sharing" experience even if they do not attack.

Yes, I know, it is no "proper" sharing but I think in this hidden single mode you will get much more points of experience to be able to survive.

One of users on grimrock.net forum has found this possibility. :D

It is spoiler so I'm including this info as attachment - download, rename it and open in text editor to read.

Here is a screenshot "how proper solo mode should look like" if you are playing it correctly.

Of course it can be counted as spoiler information so be warned ;)