I must tell "The life of virtual host name of 2ch" to you. It is a little bit long, but it is very important. Please read carefully and please understand well.
1) In the beginning
It is fresh virtual host, it is brand-new.
2) Growing up
Users make threads, write many reses, discuss and sometimes flames. So, the bulletin boards prosper, and lands are made.
3) Refreshment operation
As the result, each virtual hosts have so many kakologs. Then, the cost that each server uses to provide kakologs grows bigger and bigger.
Then we carry out refreshment operation to divide it into another server. The refreshment operation is as follows:
a) _add_ a different virtual host at the current server usually in 2ch, it is "the increasing name", e.g. for society5, 2ch will add society6 in the _same_ server hardware.
b) all of live threads are moved (called "Iten") to new virtual host, but all kakologs are NOT moved. they are left in old virtual host.
c) after that, we move old virtual host to kakolog server, it has very disk space. 2ch already has two kakolog servers, oyster902+Suma and banana3000. They are specially tuned for providing kakologs.
d) as a result, we can divide the kakolog load from live servers.
4) End of Life
Time goes by, and server greets its life. Any kind of high-performance server must come to retire itself in several years, so, the progress of the hardware is amazing.
It is time of this, we moved all of live thread to brand new servers. And we move a dead body of virtual host into kakolog servers.
This is the cruel theory of 2ch. We have already have a dead body of a lot of virtual hosts in oyster902 and banana3000. The number of over 200, please see http://mumumu.mu/serverlist.html at oyster902 section. 2ch gives the name for oyster902 and banana3000 to "memories.2ch.net" and "memories2.2ch.net", with affection for the past.
So, pie, sakura01, sakura02 and sakura03 are old. I think we would operate 4) in the above.
Mochiron, we can set two or more virtual hosts per server without any additional IP address if it is technically needed, it is technically very easy, only Apache's matter.
Really, in 2ch, we already set virtual hosts more than 2 to some servers. e.g. banana3206 have three virtual hosts, academy6.2ch.net, science6.2ch.net, and society6.2ch.net. These virtual hosts have different board settings, for good BBS operations.
so, if we needed two or more virtual hosts per server technically and operationally, we can _add_ more two virtual hosts _atode_. As I wrote before, it is technically easy.
That's all. I believe our tomorrow is better than today. Let's play a game together.
I don't know any not technical matters (financial, and so on) of bbspink. If your request about four virtual host is derived from not technical reason, it is completely my outside.
I only help you from a technical aspect of BBS operation, because I am engineer.