#1
I own some Roland elec drums and would like to achieve two things:

1. Play along to backing tracks
2. Have the option of creating a 'virtual studio'. By that I mean the drums, guitar, bass (and vocals?) plug into some sort of device which then mixes it, allowing volumes to be balanced, and then outputs to four headphones

Is this four track territory? Sorry, I don't know much about this and even less about manufacturers and brands.

Any help would be appreciated.
#2
The main thing I guess is if it needs recording capability or not.
The simplest would be some kind of multitrack standalone with connection capability, say Zoom R series, R16 probably or R4 so you can get bigger track count.
Then headphone amplifier like this
http://www.sweetwater.com/store/detail/HP4
Or another brand.

There's also the PC solution, getting multichannel interface that allows realtime monitoring, one of the Focusrite or Presonus 8 input interfaces and coupling it with headphone amp again. That one will need to be pc connected all the time.

The cheapest alternative is probably a mixer that can handle the amount of inputs you need and headphone amplifier. It could have usb recording option to Pc or you can couple it with small idea recorder to record everything in stereo, one of the Zoom, Roland, Olympus small recorders. In my case that's how I record practice - Behringer Eurodesk into Boss Micro Br, both together about $150 on the used market. In your case you still need headphone amp to split the signal into 4 headphone outputs.

Last but not least there are all in one headphone practice solutions, look at this:
http://www.sweetwater.com/store/detail/JHTourBus

Roland is also rolling out competing product to the Tourbus which would even have some guitar multifx built in.