Well, there are no options to limit the maximum length nor the character input to numbers only, but it's possible with a small custom script without touching the plugin code.
Put this code into the functions.php file in your active theme directory (start copy from line 3):
This will execute a small code snippet on page load, which validates the input for number only, and maximum length of 4.
You also mentioned, you don't need the magnifier, you can hide that on the Theme Options -> Search Text button panel: https://i.imgur.com/iWE3bAB.png
(I've placed that option accidently to the wrong panel)
I hope this helps!Best,
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