[Python] Python + Polar: Split a column of string into list of list

I'm new to python polar library, may I ask how could I split a column of string into list of list?

Minimum example:

df = pl.DataFrame({'test': "A,B,C,1\nD,E,F,2\nG,H,I,3\nJ,K,L,4"})


I try the following, somehow I stop after the first split

df = df.with_columns(pl.col('test').str.split('\n')


My desired result would be it return a list of list, and also convert the forth position of every sublist into Int64

[["A","B","C",1], ["D","E","F",2], ["G","H","I",3], ["J","K","L",4]]

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