HP4 : Cheap and cheerful

This does as it needs, it amplifies 4 headphone signals as need and does a very good job at that. Its pretty well featured too, able to take 4 mono or stereo signals and instead of passing them through to the corresponding output channel, each output can choose which signal from the input it outputs. This is useful if your mixing 4 monitor mixes as you can switch between the channels and hear what the artist is hearing.

It also has a line but I cannot comment on the usability or functionality of this as I have yet to use it.

It doesn't have the most amazing build quality, it does indeed feel cheaper than other units and on channel one my units volume knob is extremely stiff; initially taking two hands to turn it.