For the UK crowd we can also use the 3 phase option. Our domestic 240v supply is 1 hot wire at 240v, 1 neutral and an earth, but we also have an industrial 3 phase supply that is 3 hot 240v lines wired in star or delta with a common ground. The voltage between any 2 live wires is 400v.
So we can buy industrial 3 phase elements that have 3 heater cores in the same base, typically this is a 2.25" BSP threaded unit but they are also available with a 2" thread so don't order the wrong one.
If you were to buy a 12kW 400v 3 phase element and wire it up to a 240v single phase with all 3 heaters in parallel, it would convert it into a 4.3kW element pulling about 18Amps.
Now this is a higher total current than a conventional 4kW single phase element, but the current in each heater core has been reduced to just 6 Amps, so each element individually has a far lower watt density.
You can get them here: http://www.milkingmachines.co.uk/uk2shop-11.htm