Seems like you might be in Europe based on listing European sizes. Altama (https://originalfootwear.com/) seems to make boots down to men's size 4 and they ship internationally. Mostly combat boots/police boots. They make boots for the US military. They're not Goodyear welt, so you might not be able to have them resoled, but they should last for a while. When I used to wear Altama boots I had to size down a little too.

Drew's Boots makes boots down to a women's 5, which would be a men's 4. More expensive but a different style and more BIFL.

Women's Corcoran jump boots go down to size 4. They have some other styles down to a women's 4, but it seems kinda random as to what they make that small. Their factory store doesn't ship outside the US, but you should be able to find the same boots getting sold by a retailer that does.