i have a back condition, so i sometimes encounter this problem.
like stacy said, laying on her front either with or without a couple of pillows under her hips should help ease the pressure.
being on all fours should be mostly ok, but its probably better if she can keep her shoulders up rather then arch her back down to rest her upper body onto the bed.
on her back with you on top would normally be ok, although depending on your height, she may be more comfortable with a pillow under her hips, however lifting her legs up and onto your shoulders may put added pressure on her lumbar region, especially as it can curl the area and leave it unsupported.
her on top may be ok, but you could also find that it puts a strain on her back, especially her lumbar region, as she uses her core muscles hold herself up and gyrate around on top of you!!
of course, everyone is different, and the best way to decide what is ok for her and doesnt cause her any pain, is to ask her!!
i hope this has been of some help
have lots of fun!!