How to mirror an object in a 90 degree angle? So, I'm building a TARDIS...

I'm working on the details off the side, and want to then mirror it to the other side(the one adjacent to the one already detailed), so I don't need to do the door two times.
How can I do this?
You have to use an array modifier to rotate it 90 degrees, then mirror it along the X and Y axis:



Result: