I'm using Linux servers
but now i want to use FreeBSD, it seems to be very nice,stable, .... . and i must do the migration.

But on linux, i used /etc/inittab for having some applications with "/sbin/init 4" and "/sbin/init 3" for others.

How can i do to do like this with FreeBSD ?

Voodoo

Well I think man init is worth looking at but I think the answer is "freebsd isn't linux"(*) as in there isn't an equivalent to those runlevels. Michael Lucas has an article in SysAdmin that explains FreeBSD and runlevels:

FreeBSD doesn't have runlevels. If you need to work on the system before it completes booting, you can set the system to boot only in single-user mode (see above). In single-user mode, the / filesystem is mounted read-only. No fsck has been run, other filesystems are not mounted, and swap is not active. Essentially, it is the earliest point where the system can give you a shell prompt. During single-user mode, you can choose exactly which system components you want to use. All the commands you need for a complete boot are documented in /etc/rc. A ^D or exit in single-user mode will continue the boot process and bring the system up normally.
http://www.samag.com/documents/s=116...003a/0003a.htm

Hopefully it is ok to paste that much of the article.

So now the question becomes what are you doing differently in in runlevel 3 and 4 and how can this be accomplished under FreeBSD? I'm sure someone will have suggestions if you can detail your needs.

Oh, and for anyone wondering why I link to so many Michael Lucas articles, it's just because they are there! He has written quite a bit and all of it is informative and lots of it comes up near the top of the results when searching for FreeBSD topics (the article able was in the top 10 when I did a google search for "freebsd runlevels"). He writes some kickass stuff.

* Note: I don't mean this to be snotty or anything. Just pointing out that there are differences and this is one of them. I'm in no way a zealot of any particular OS although I do have my preferences.

I don't see that as being snotty. There are a few things that users coming over from Linux into the BSDs will find puzzling, and that's the easiest way to put it. Just like windoze users coming into the Linux camp; linux isn't windoze (thank heavens).