You can rearrange columns directly by specifying their order:
df = df[['a', 'y', 'b', 'x']]
In the case of larger dataframes where the column titles are dynamic, you can use a list comprehension to select every column not in your target set and then append the target set to the end.
>>> df[[c for c in df if c not in ['b', 'x']]
+ ['b', 'x']]
a y b x
0 1 -1 2 3
1 2 -2 4 6
2 3 -3 6 9
3 4 -4 8 12
To make it more bullet proof, you can ensure that your target columns are indeed in the dataframe:
cols_at_end = ['b', 'x']
df = df[[c for c in df if c not in cols_at_end]
+ [c for c in cols_at_end if c in df]]