All routers and devices are usually dual stack. This means if you transmit IPv4 packets, the IPv6 device will negotiate this and receive as IPv4 and send as IPv4 so the end point receives it correctly.
There are no devices, that I know off which are just IPv6, this would be stupid and the vendor wouldn't sell any devices.
IPv6 through IPv4 to its end point of IPv6 needs to encapsulate the IPv6 packets in a IPv4 packet. This can be done with tunnelling (manual IPv6 tunnels, 6to4 tunnels or ISATAP), toredo tunnelling or a 3rd party broker service. Each have advantages and disadvantages.
The information has different header information so that the IPv6 devices know that it is encapsulated and can be open correctly at each end. The header is usually stripped off when the encapsulated packet is received by the IPv6 device. IPv6 header infromation is completely different to IPv4 header information. IPv6 headers have less information now.
Every operating system is dual stack now, except Win XP, which you can download a client to deal with.
The various tunnelling methods are a little too long to describe in this post.
To answer your question there is no need to tunnel from a IPv4 device through a IPv6 device to a IPv4 end point, because the middle man will be dual stack. This means IPv4 and IPv6.