How do I mount a ceiling box in 4" thick ceiling?
I agree with answer #1 with the following exception:  Do not use longer screws to attach the fixture.  Find a suitable box with extensions as needed to make the box opening flush with the ceiling surface.  You do not want a gap between the box and fixture the could allow heat and spark from a fault in the box to escape. Here are products that I would use in your situation.  These may not be available at home improvement store but are available at electrical suppliers who service contractors.


For support between studs -- Erico/Caddy TSGB24 or simply use a 2x4
Box attached to support --4" octogon box
Two extension boxes as need to bring opening flush -- 4" octogon extension box 2 1/8" deep 


This combination gives you a box 5 3/4" deep.  Adjust the support on the attic studs to make this flush with ceiling.

Remember that it is important to not leave a gap that could cause a smoldering fire in the voids of the ceiling structure.  This is the reason that most electrical codes require that box faces be flush with the finished surface.