How to Select the First n Columns of a Pandas DataFrame

How can we select the first n columns of a Pandas DataFrame?

Suppose we have this DataFrame df.

     x1   x2  x3  x4  y
0     7  143  22  23  1
1     1   23  22  29  0
2     3  133  24   0  1
3     1   39  27  23  0
4     0  137  40  23  1

If we only want the first 4 columns, then we can either slice by the actual index, or use -1.

df.iloc[:,:4]
df.iloc[:,:-1]

If we only want the last column (index of 4), once again, we can either slice by the actual index, or use -1.

df.iloc[:,4:]
df.iloc[:,-1:]