If you found a tankless that required a 60A circuit, it would not draw that, and figure around 75% of that which equals around 11K BTU. If you divide that by 60 to get a minute's worth of energy use, that's about 180 BTU. One BTU will heat one pound one degree. 4G of water/min is about 32#, so divide the available 180BT/32 and you'll get all of about 5.6-degrees rise. Now, if I did my math right, you can see why electric tankless systems need LOTS of power. A commonly sized gas tankless would have almost 20x more energy to put into warming the water. Now, I'm not sure of the sizes of electric tankless systems out there, but one that would work on a 60A circuit might work to make warm water for a sink, not to fill a tub at any decent rate. Now, if you wanted to wait, lower flow means greater temperature rise, but then you have to consider how much what's already in the tub would cool off while you were filling it so slowly.