I have a column of 0s and 1s. I only want to keep 1s in the output column only if they end up being at least 4 rows apart.
Note that simply doing diff() is not a solution, because this would eliminate too many 1s. Here’s an example:

df = pd.DataFrame.from_dict({'ix':list(range(12)), 'in':[1, 0, 0, 1, 1, 0, 1, 0, 1, 0, 0, 1]})
df['out'] = [1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0] # desred output

    ix  in  out
0    0   1    1
1    1   0    0
2    2   0    0
3    3   1    0  # this 1 needs to become 0
4    4   1    1  # we keep this 1, because the previously kept one is sufficiently far
5    5   0    0
6    6   1    0
7    7   0    0
8    8   1    1
9    9   0    0
10  10   0    0
11  11   1    0

Intuitively it seems like it should be solved with some combination of grouping, diff and cumsum(), but I haven’t been able to figure it out.