Yes, you can use Virtualbox Headless. I will give you an example configuration:
VBoxManage createvm --name "ubuntu-server" --register
VBoxManage modifyvm ubuntu-server --memory 1000 --vram 64 --acpi on --cpus 1 --ostype Ubuntu_64 --pae on --hwvirtex on --vtxvpid on
VBoxManage modifyvm ubuntu-server --nic1 bridged --bridgeadapter1 eth1 --nic1 bridged -- bridgeadapter2 eth0
VBoxManage createhd --filename ubuntu-server-disk.vdi --size 2000
VBoxManage storagectl ubuntu-server --name "IDE Controller" --add ide --controller PIIX4
VBoxManage modifyvm ubuntu-server --hda ubuntu-server-disk.vdi
VBoxManage storageattach "ubuntu-server" --storagectl "IDE Controller" --port 0 --device 1 --type dvddrive --medium /home/mydir/ubuntu-10.04-server-amd64.iso
This example is copy paste from my last configuration on my own virtual server. Then you start the server like this:
VBoxHeadless -s "ubuntu-server"
You will be given a port to which you can connect or you can use something like NXclient/server to connect graphically through ssh.
Be aware that maybe some of my commands may not be correct since I can't really remember which version of Virtualbox I used when created the machine (it runs for last 2 years now), but download their pdf manual from virtualbox site, it's very,very informative, all the options are well presented there.
As for stability and usability, I run headless virtual machines Debian Lenny, Ubuntu and Windows, Linux machines can last more than 100days without any problems, haven't tested windows that much, but I'm sure it shouldn't do bad as well.
As for 4 different IPs attributed to different machines, I believe it's more matter of network setup, every one of virtual machines if set in bridged mode are in LAN, which means that you need to route the traffic through to these machines from outside, it's like having couple real computers connected in one LAN. Since I don't know your network setup, I can't really say, and couldn't probably say much since I never had the fortune of having more than one external IP.