"How does the 4th proviso to Section 20 of IGST Act, 2017 impact GST interpretation?"

Understanding 4th proviso to Section 20 of IGST Act,2017 which  is extracted below for reference:

"Provided also that in case where the penalty is leviable under CGST Act and SGST Act,the penalty leviable under this Act shall be the sum total of the said penalties ".

This proviso can be explained like this.

If maximum general penalty is Rs 25000 each under Section 125 of CGST Act and SGST Act  where no separate penalty is provided for , then maximum general penalty imposable for violations of provisions only of IGST Act would be Rs 50000 which is sum total of the said penalties under CGST and SGST Act.

Further,  if specified 10% penalty of Rs 1 lakh  is imposed each on  short payment of CGST of Rs 10 lakhs  and  SGST of  Rs 10 lakhs,  under Section 73(9) read with Section 122(2)(a) CGST Act/ SGST Act , then penalty imposable in case of IGST short payment under Section 73(9) read with Section 122(2)(a) and Section 20 of IGST Act 2017 would be Rs 2 lakhs where IGST demand is for Rs 20 lakhs.

Let us take another example to interpret 4th proviso to Section 20 of IGST Act.

If IGST short paid is say Rs 150000 , then penalty imposable is Rs 20000 and not Rs 15000 as the minimum penalty under Section 73(9) r/w Section 122(2)(a) of CGST Act/ SGST Act,2017, is 10% of the tax short paid or Rs 10000 whichever is higher. 

Thus, sum total of minimum penalty would be Rs 20000 on short payment of IGST of Rs 150000 in view of 4th proviso to Section 20 of IGST Act 2017.