Let say i want to use it in parallel. Does it mean it use a current 4.2A and i have to supply 2.73V ? I don't know which supply voltage(s) I can take.
If you supply it with 2.73V it will take 4.2A because the coils have a resistance of .65 ohms (Ohm's law!). In practice this is almost never done and usually the motor is driven at higher voltages, commonly up to ~40V, with a driver that limits the current to the motor ("chopping" drive). The advantage of a higher voltage is faster response from the stepper and higher peak speeds.
then, what is he difference between series and parallel? The torque stays the same with less current. sou wouldn't it be obvious to use it in series?
Less inductance when wired in parallel thus the motor can reach higher speeds for a given drive voltage. Also, the torque listed is the holding torque; when the motor is spinning the torque drops off rapidly with higher speeds. A lower inductance motor or higher drive voltage helps you keep torque at higher speeds.
...the problem is that it only can handle currents till 2A. in fact I can't find any drive which can handle this currents...
Try eBay for a "TB6600" driver which is capable of up to 5A. ~$30. The current should be settable either by dip switches or an onboard pot.