Combine rows of pandas dataframe if values from multiple columns are the same and combine strings from one column [duplicate]

So I have this pandas dataframe coded in a python jupyter notebook as an example:

col1|col2|col3|col4
-------------------
"a" |"b" |0   |"d"
"a" |"b" |1   |"e"

I am trying to combine rows into one that has the exact same values in col1 and col2, and concatenate col4 values. This would be the intended result:

col1|col2|col3|col4
-------------------
"a" |"b" |0   |"de"

I have tried using the pandas aggregate function but I am still not getting what I want by concatenating string from col4.

Thank you so much guys for your help!