I'm having trouble understanding your desired farm
topology. Here is what you wrote:

Server1: domain controller
Server2: database server
Server3: main MOSS server
Server4: MOSS farm

I have no idea what you mean by "main MOSS server" and
"MOSS farm." Please turn off your CAPS LOCK key and
explain what you would like to do with Server3 and
Server4.

(Although I can't predict your answer, I doubt you'll
need to setup Microsoft Cluster Service at any point)

I'm guessing he means load balanced web front ends, in which case no
clustering is needed, only network load balancing.

To setup network load balancing, read this forum

www.sharepointu.com/forums/p/1093/2827.aspx#2827

i''l try now to
explain the reall world for my topology.
Server 1: Domain Controller.
Server 2: SQL Database.
Server 3: MOSS Server.

Now, i have everything is installed and up and runing.also i have some websites
is working in this MOSS Server.there is no problem till now.
In Future, i want to add a new MOSS server  to this topology and make the server
farm between this two MOSS Servers.
I know how to do that and how to install MOSS in Server farm.
But my question is: after i install the second MOSS Server and make the Farm
between this two Servers.
Is my Sites it'll be there by default and still runing.
Also, the important question is: what if my MOSS Server ( Server 3, the 1st one
i installed ) is Down, and the second one is up and runing,
Is my sites still runing and all SharePoint Services still working or no, and
how the URL it'll work.
Because see: lets say the URL is: http://MOSSEP/Sites/DMO , thats mean the
server name is MOSSEP.
But now this Server is down, So how my sites it'll come and work.
I hope both of you can understand me now and waht exactly i need to know.

What you are talking about is network load balancing - in short your
site will be accessed via http://thebalancedname/ which is an alias for
the load balanced address rather than the individual name of your
machine. Brad's link has the specific s of how to set this up. If the
URL name needs to change, that can be done at the time you add your new
machine via a change in the Alternate Access Mappings (AAM) settings.

It sounds like you want
total redundancy between your two moss  servers. I
have not set this up before, but I'll mention my
thoughts. Hopefully someone else can respond with
tried-and-true method.

When it comes time to add your 4th server, add it to
the farm  and install all MOSS components onto it.
Setup Network Load Balancing between the 2 servers.
Configure an SSP on the 4th server, but do not make it
the default. If the 3rd server  dies, change to the
new SSP and restore your most recent backup of the
mysites site collection to the 4th server.