使用pandas，我想为一组长度为4个或更多字符的名字派生一个姓氏列。
我试过这些：

data = pd.read_csv("Data.csv")
data

#split the EmployeeName into firstname and lastname

flname = data['EmployeeName'].str.split(expand=True)
flname

#add first name column to data frame
data['FirstName'] = flname[0]

#apply condition on first name
dfname = data['FirstName'].apply(lambda x:x if len(x) \> 4 else None)
dfname = dfname.dropna()

#add last name and new first name columns to data frame
data['LastName'] = flname[0]
data['NewFirstName'] = dfname
data

#This is the wrong bit that throws an error
derived_name = data.apply(lambda x:x if data\['FirstName'\] in data\['NewFirstName'\] else None)
derived_name.dropna()

TypeError：unhashable type：'系列'

是否有更短的方式来使用pandas编写这些代码行？