I've been looking for something like that for a while, and no luck. There's really no guitar level line mixer that I know of that accepts more than one input. You could bet a little berhringer mixer (I used one to mix a synth into my bass amp, which has a single input). There's also the little mixer that diamond pedals makes for use with the memory lane delay, but that only gives you one extra input (and you can use the pan knob as a level).
I wish someone would just make the damn thing! 4 in 1 out mono guitar level line mixer. Volume knobs. Small! Tuner out!