Generally I want to know when and where to use IPv6 and also if it is necessarry for each server to support both or if they can support only IPv6.
Obviously we would need to make sure the ISP and routers are set up for IPv6 first.
If you mean by 'equivalent' that the server sitting on ipv4 only can be reached from ipv6 network, it can: 192.0.2.128 is reached from ipv6 networks by visiting ::ffff:192.0.2.128
If you want to have the server sitting on ipv6 it's a bad idea like mrdenny says, but it's technically (and very awkwardly) possible with methods explained on http://www.sixxs.net/tools/gateway/ : access the ipv6-only http://www.kame.net/ by appending .ipv4.sixxs.org to it = http://www.kame.net.ipv4.sixxs.org/
The turtle is dancing only on ipv6 connections :)
So yes it's possible to have the server on ipv6 only, but you would definately want to go for mrdennys solution.