Creating a never-repeating random hexadecimal generator

My company has asked me to develop a 4-byte random hexadecimal generator for some security hardware we have, however I am new to this sort of thing. (To clarify, each code consists of 4 hexadecimal digits).

We currently have a list of 124 hexadecimals that are in use and I need to be able to not repeat those hexadecimals as well as ones that will be generated with this program. I have developed a GUI and a generator, I just need help making sure it never repeats.

This is what I have so far:

# random hexadecimal generator engine 
text = "" 

for i in range(4): 
    rand = random.choice('0123456789abcdef') 
    text = text + rand 
    print(text) 

# creating the window 
root = Tk() # GUI window 
root.title("Hex Generator") 
root.geometry("250x150") 
app = Frame(root) 
app.grid() 
label = Label(app, text="Click to generate a Hexadecimal") 
button1 = Button(app, text="Generate") 
label.grid() 
button1.grid() 
label1 = Label(app, text=text) 
label1.grid() 

# kicking off event loop 
root.mainloop()