Assuming this is your driver, on that page scroll down and it is written "Current Limit = VREF × 2". So start with 0.8A peak, that means Vref=0.4V. It should have enough torque like that, but if it does not then you can increase to 1A. For increasing setting watch the driver temperature and heatsink if possible.
In relation to psu, if you have one arduino mega i guess that should be budgeted to like 1A, and 4 drivers perhaps budget 1A each. Meaning the psu of 5A may run near the limit, but perhaps manageable. Even if it has 5A label does not mean it will provide 5A exactly, or maybe it does but will be running very hot. So you will have to test and see. But the other psu with 2,5A seems like its not enough. I assume you dont need extruder or anything else, otherwise you need to budget current for that also, and then 5A will not suffice.
In relation to psu, if you have one arduino mega i guess that should be budgeted to like 1A, and 4 drivers perhaps budget 1A each. Meaning the psu of 5A may run near the limit, but perhaps manageable. Even if it has 5A label does not mean it will provide 5A exactly, or maybe it does but will be running very hot. So you will have to test and see. But the other psu with 2,5A seems like its not enough. I assume you dont need extruder or anything else, otherwise you need to budget current for that also, and then 5A will not suffice.