If you use the "Reply to Thread" button, rather than "Post Quick Reply", the edit window has a button marked with a "#" symbol. This will wrap selected text in code tags. For example:
Once you've seen the "code" tags in a post, you can also enter them by hand. I'm assuming the second command was actually "whereis xvnc4viewer".
which xvnc4viewer yields "xvnc4viewer: command not found"
whereis xvnc4viewers yields "/usr/bin/xvnc4viewer /usr/bin/X11/xvnc4viewer"
file /usr/bin/xvnc4viewer yields "xvnc4viewer: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.24, BuildID[sha1]=0x2b92ab1fc9ce35edb8e6fffef5da4981ca2828f3, stripped"
echo $PATH yields "/usr/local/phenix-1.8.2-1309/build/intel-linux-2.6/bin:/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games"
As far as the issue is concerned, the key seems to be that "which" doesn't find the program but "whereis" does.
From man which
From man whereis
which returns the pathnames of the files (or links) which would be exe‐
cuted in the current environment, had its arguments been given as com‐
mands in a strictly POSIX-conformant shell. It does this by searching
for executable files matching the names of the arguments. It
does not follow symbolic links.
Perhaps you could try using the guest account on your machine and see if it works from there? If it does, I think that would indicate that the path ($PATH) in your account is the problem. There's nothing that I can see in your output that indicates that, so something else must be at work here.
whereis locates source/binary and manuals sections for specified files.
The supplied names are first stripped of leading pathname components
and any (single) trailing extension of the form .ext, for example, .c.
Prefixes of s. resulting from use of source code control are also
dealt with. whereis then attempts to locate the desired program in a
list of standard Linux places
Another suggestion would be to try re-installing the package.
sudo apt-get install --reinstall xvnc4viewer