In the USA, I am replacing a 4x 30amp tankless water heater on double pole circuit breakers with one that is 1x 60 amps. Can I reuse 2x 30amp double pole circuits and wire them both to the L1-L2 terminals in the heater? I want to avoid pulling new #6 wire that the 60amp DB-CB requires.
No you cannot do that. The circuit breaker is there primarily to protect the conductors from overload. So any conductors connected to your 60A breaker must be conductors rated for 60A even if the load it serves is less than 60A.
So even though your intent to parallel up two conductors rated for 30A each seems like it would work you do not want to play games with safety. One of the paralleled conductors could become disconnected or damaged leaving the other conductor to have to bear the brunt of the load which would overrate it and potentially cause overheating, melting of insulation or even fire.
Globally, no. You cannot "parallel" except in extremerly rare circumstances and with special equipment.
But consult the documentation on that water heater. You can do that if and only if the labeling or instructions say so, and by following the instructions. This is codified in NEC 110.3B.
(I'm hoping that the heater is actually two 30A heating elements internally, and some jumper straps could be removed to allow them to be powered separately).
Given the annoyance of having to change the wiring like that, I would return that water heater and get one that takes 2-4 30A circuits.
Also be aware you are halving the size of the on-demand heater. It will give you dramatically less hot water. A huge risk with on-demand water heaters is people failing to adequately size the heater, and then being disappointed with the results and blaming the heater. This often leads to failed installations (people removing it in disappointment and having to buy another one).