is it possible to get 20 volt 2a from 2 powerbank which have 5v ,2a 4x output?
Is it possible to get 20 volt, 2A from 2 power bank by connecting 4 x USB ports, each rated at 5V, 2A    
  
  Is it possible to connect 4 x USB 5v, 2A to get 20V at 2A
  or 4 x (USB 5V at 2A) to get 1A current (parallel circuit and buck)


You can 


Use completely separate power banks connected in series in series to get higher voltages.
eg 4 x (USB 5V, 2A) powerbanks may be connected in series to get 20V at 2A.
Usually (!) connect multple 5V ports on the same power bank to get the sum of the current ratings, but only at the same voltage.
eg a powerbank with 4 x 5V at 2A ports can (usually) have the 4 ports connected in parallel to give 5V at 8A.
Note that it is technically possible that this will NOT work in some cases , but it usually will. 


You cannot connect multiple ports on a single power bank to obtain increased voltage. This is because on power banks with multiple "USB" 5V output ports the negative grounded sides of all the ports are connected together. The ports MAY be separate outputs internally but commonly will simply be parallel connections to the same switching converter.
Either way, you cannot connect them together to get more voltage.

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Boost Converter:

You CAN use a boost converter to increase the avai;lable voltage from one or more power banks, with some loss on power avai;able due to inefficincies in the converter.
For example you might use any of the following powerbanks
- 5V with 1 x 4A output
- 5V with 2 x 2A outputs
- 2 x (5V at 2A)

to provide a 5V at 4A input to a boost converter.
The 5V x 4A = 20 Watts input power would probably provide around 16 Watts +/- 2 Watts using typical boost converters.  

In addition the 2 x (5V at 2A) powerbanks could be used in series to make a 10V x 2A supply to power a boost converter. This would possibly be slightly more efficient than a converter running on 5V input.