If you don't mind clearing ser passwords, then I would export users from 10.4 and import into 10.8
There are some issues with service ACLs in doing this, but its still the fastest process.
If users are allowed to set their own PW, the you give provide preset pw's (either unique or common) and a URL to allow users to reset their PW.
If you need to retain passwords, what I would do is clone the 10.4 server, then upgrade it all the way to 10.8 then archive OD from that and import into a clean-install of 10.8 server.
Whataver you do, don't rely on a 10.4 to 10.8 migration, you'll want a clean 10.8 install.
The offline 10.4 -> 10.8 would allow you to retain PWs, but it creates alot of extra work for you.