tankless water heater with four lugs
It appears  Circuit B powers nothing but heaters 3 and 4, so consult with the factory. It may be possible for the unit to happily run with circuit A only, at the 60A power setting. 

Other than that, you will have to find 2 spaces for another 60A breaker.  This appears to be a GE Qline which supports their advanced design for "double-stuff" breakers, including 2-pole double-stuffs that straddle 2 spaces (as in right above the 60A). Convert 4 spaces worth of breakers to double-stuffs and you'll have the room. 

Now the bad news.  The wires seem to be run in 1/2" EMT conduit, with individual THHN wires, presumably 6 AWG.  They didn't need a ground wire, but they ran one, so the wire count would be 3 instead of 2 - it's a conduit fill rules thing.  Regardless, this 1/2" conduit cannot hold four #6 wires.  For that, you'll need 3/4" conduit, and will just fit with that one #10 ground.   So you will have to re-run all this conduit.  

When you buy wire, use two black and two red.  The blacks are circuit 1, and the reds are circuit 2.  For each circuit, the two hots are interchangable - no value in phasing them - but you certainly do need to tell the circuits apart!   

Now if you run 1" or 1-1/4" conduit, you can run #4 aluminum wire.  No idea if that's any kind of savings for you.  Whether the tankless can accept #4 or aluminum is a factor; otherwise you'd have to pigtail with expensive Polaris connectors. 

Given that, you might as well rethink the location of the heater. Very often, these on-demand heaters are slapped down exactly where the tanked heater had been. That location wasn't optimal at all. It was put there because tanked heaters are huge and ugly, and often because a gas vent is possible there.  It's usually not the ideal location for a small tankless.  You want that as close as possible to the most important point(s) of use, e.g. the place you least want to wait, wait, wait for hot water.  For me, that's the shower.