Here's a 4" sq., but it's a cheesecake pan (which just means it has a removable bottom): http://cooksdream.com/store/fdpsqcc442.html
I use their 3" round cheesecake pans for mini cakes and they work great. Never tried the square ones though.
Magic Line makes a 4 1/2 pan.
http://www.countrykitchensa.com/catalog/product.aspx?productId=612259
Why not just bake sheet cakes or larger squares and cut them down to 3, 4, or 5" squares using a ruler.
I have ML pans in 3in and 4in and they work great. Very clean lines and they cook evenly. I found mine at a local cake supply store but you should be able to find them online. Hope this helps!
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