enter special character in between text in python at after certain length

Question

code is running good in main project , took some sample code from that as below

from tkinter import *

root = Tk()
root.geometry('200x200')
root.title('Testing')
num1 = StringVar()
num2 = StringVar()

def callback(integer):
    if integer.isdigit():
        return True
    elif integer == "":
        return True
    else:
        return False

def limit_no1(no):
    c = no.get()[0:16]
    no.set(c)

num1.trace("w", lambda name, index, mode, sv=num1: limit_no1(sv))
reg = root.register(callback)
ent1 = Entry(root, textvariable=num1)
ent1.pack()
ent1.config(validate="key", validatecommand=(reg, '%P'))

this has some condition as accepting only numbers and up 16 digits but as like some web apps i need spacing after 4 characters is there any method for that so that i can apply to this and and for input of currency also as ',' in btw data

I worked on some code but indexing getting issue in indexing

the below code continuation of above only

num2.trace("w", lambda name, index, mode, sv=num2: limit_no2(sv))
ent2 = Entry(root, textvariable=num2)
ent2.pack(pady=20)
ent2.config(validate="key", validatecommand=(reg, '%P'))

def limit_no2(no):
    c = no.get()[0:19]
    split_string = [c[i:i + 4] for i in range(0, len(c), 4)]
    if len(split_string) > 1:
        final_string = ''
        for i in range(len(split_string)):
            final_string += split_string[i]
            final_string += " "
        no.set(final_string)
    else:
        no.set(c)

root.mainloop()