Python: Determining period of the day based on hour using a for loop and conditionals

I would like to name the period of the day based on hourly information to my dataframe.

For this, I am attempting the following:

day_period = []

for index,row in df.iterrows():
        
    hour_series = row["hour"]
    
    # Morning = 04:00-10:00
    #if hour_series >= 4 and hour_series < 10:
    if 4 >= hour_series < 10:
        day_period_str = "Morning"
        day_period.append(day_period_str)
    
    # Day = 10:00-16:00
    #if hour_series >= 10 and hour_series < 16:
    if 10 >= hour_series < 16:
        day_period_str = "Day"
        day_period.append(day_period_str)
        
    # Evening = 16:00-22:00
    #if hour_series >= 16 and hour_series < 22:
    if 16 >= hour_series < 22:
        day_period_str = "Evening"
        day_period.append(day_period_str)
        
    # Night = 22:00-04:00
    #if hour_series >= 22 and hour_series < 4:
    if 22 >= hour_series < 4:
        day_period_str = "Night"
        day_period.append(day_period_str)

However, when double-checking if the length of my day_period list is the same as that of my dataframe (df)... they differ and they shouldn't. I can't spot the mistake. How can I fix the code?

len(day_period)
>21882

len(df)
>25696

Here's a preview of the data:

    timestamp               latitude    longitude   hour    weekday
0   2021-06-09 08:12:18.000 57.728867   11.949463   8   Wednesday
1   2021-06-09 08:12:18.000 57.728954   11.949368   8   Wednesday
2   2021-06-09 08:12:18.587 57.728867   11.949463   8   Wednesday
3   2021-06-09 08:12:18.716 57.728954   11.949368   8   Wednesday
4   2021-06-09 08:12:33.000 57.728905   11.949309   8   Wednesday

My end goal is to then append this list to the dataframe.