Q:

Create a column based on presence of dates within a specific range in multiple other columns

I have a dataframe called df that looks similar to this (except the number of 'Visit' columns goes up to Visit_74 and there are several hundred clients - I have simplified it here).
Client    Visit_1     Visit_2     Visit_3     Visit_4     Visit_5         
Client_1  2016-05-10  2016-05-25  2016-06-10  2016-06-25  2016-07-10
Client_2  2017-05-10  2017-05-25  2017-06-10  2017-06-25  2017-07-10
Client_3  2018-09-10  2018-09-26  2018-10-10  2018-10-26  2018-11-10  
Client_4  2018-10-10  2018-10-26  2018-11-10  2018-11-26  2018-12-10

I want to create a new column called Four_Visits with two values, 0 and 1. I want to set Four_Visits to equal 1 if there are at least four dates in any one of the columns from Visit_1 to Visit_5 that fall between 2018-10-15 and 2018-12-15. The resulting dataframe should look like this:
Client    Visit_1     Visit_2     Visit_3     Visit_4     Visit_5     Four_Visits  
Client_1  2016-05-10  2016-05-25  2016-06-10  2016-06-25  2016-07-10  0
Client_2  2017-05-10  2017-05-25  2017-06-10  2017-06-25  2017-07-10  0
Client_3  2018-09-10  2018-09-26  2018-10-10  2018-10-26  2018-11-10  0
Client_4  2018-10-10  2018-10-26  2018-11-10  2018-11-26  2018-12-10  1  

A:

Convert to datetime if not already, then use filter and >= + <= to check if more than 4 visit columns fall between the dates for each row:
import pandas as pd
# df = df.set_index('Client').apply(pd.to_datetime).reset_index()

df['Four_Visits'] = ((df.filter(like='Visit').ge(pd.to_datetime('2018-10-15')).fillna(0).astype(bool))
                     & (df.filter(like='Visit').le(pd.to_datetime('2018-12-15')).fillna(0).astype(bool))
                    ).sum(1).ge(4).astype('int')

Output:
     Client    Visit_1    Visit_2    Visit_3    Visit_4    Visit_5  Four_Visits
0  Client_1 2016-05-10 2016-05-25 2016-06-10 2016-06-25 2016-07-10            0
1  Client_2 2017-05-10 2017-05-25 2017-06-10 2017-06-25 2017-07-10            0
2  Client_3 2018-09-10 2018-09-26 2018-10-10 2018-10-26 2018-11-10            0
3  Client_4 2018-10-10 2018-10-26 2018-11-10 2018-11-26 2018-12-10            1

