Why can't I change my region back to region 4 even though it was previously set this way?

I put a disc from Thailand in my pc and was asked if I wanted to change the code. I had 2 choices (of 5) remaining to alter the region setting. I still have 1 choice remaining but I get an error telling me that the region for the drive is region 4 and to insert media of that sort even though I have a disc of region 4 in the drive. The region currently states region 3. I select Australia and get region 4 displayed but when I click OK I get the error. Any thoughts?