Or you could pop into a doors/windows supplier, they might have some spare parts knocking about. B&Q would do an M4 screw but you'd have to cut it down.
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and it will be a stainless screw, since doorframes are exposed to the weather
it is probably a Pozi screw (there will be an "X" stamped across the crosspoint drive. If no "X", it will be a Phillips). The drivers are different.
for small quantities there are ebay suppliers like Kays
edit
I just enlarged your photo and there is something stamped on the heads I don't recognise. Try a Pozi and a Phillips driver to identify which it is (might be something odd) before ordering spares
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