#UPDATED (June 2020): Introduced in Pandas 0.25.0, 
#Pandas has added new groupby behavior “named aggregation” and tuples, 
#for naming the output columns when applying multiple aggregation functions 
#to specific columns.

df.groupby(
     ['col1','col2']
 ).agg(
     sum_col3 = ('col3','sum'),
     sum_col4     = ('col4','sum'),
 ).reset_index()