samba - Mar 2013 - Samba4 server not visible from windows network

Hi people:

I recently installed a Samba 4.0.3 server in a small company. Users
are still working as a workgroup -not as a NT nor ADS domain yet-
using Windows XP and Windows 7.
Everything seems to be working fine except for one thing: Windows
clients can't find the samba server from the Windows Network Places
(My Network Places -> Entire Network -> ...). It just appears the rest
of Windows machines but not the samba server.

Samba server only becomes accesible if I try to access to it directly
using "Start -> Run -> \\netbiosname". This didn't happen
when I had
Samba 3.x.

I tried from a windows command line something like this:

C:\> nbtstat -a netbiosname
C:\> nbtstat -c

Those commands shown above give me the right netbios name and ip
address of my samba server. Even when Windows client use the IP
address of my samba4 server as a unique DNS address things doesn't
seem to get fixed. Anyway if windows client are still working just as
a workgroup I think DNS server in this case isn't important. By the
way, nobody use WINS here.

Do you know what could cause this problem?

I hope someone can give me some ideas. Thanks in advance.