it display is the numbers from 0 to 3
Well, if you set up a for loop that goes from 0 to 3 (i.e. range(4)) using the index "i" and you tell the combobox to insert the conversion of the current value of "i" to a string, why are you surprised when your combobox contains the numbers 0 through 3?

Simply opening a file and assigning a reference to the file object to the variable "tmod" does nothing more than that. It doesn't read anything. Even if it did, there is nothing in your code that does anything with the values that might be read. That fact that you've indented the for loop under the open() statement means nothing.

I think you need to start with reading about python file I/O. As a hint, you need to open the file (which you have done) and then while looping until you reach the end of the file, read each line into a string and append that string to the combobox. When you reach the end of the file, you exit the loop and close the file.