refurb ipad mini 32gb $340
refurb ipad4 32gb $450
ipad mini 32gb (gen 2) $499
ipad Air 32gb $599
given the price points, the ipad mini would be least attractive option because of it offers least bang-for-the-buck and has the most limited future potential.
if you want a full-size ipad, the ipad4 option offers better value than the ipad Air - not worth the $150 premium for the Air unless you do a lot of gaming or plan to.
i think it'll boil down to the ipad4 or the 2nd gen ipad mini. bigger is always better if you plan to use it for pdf reading; screen size is not that significant when reading epub or mobi.
the 2nd gen mini offers the most future-proof option and its lesser weight & size might could be important to you. i'll reserve judgement and wait for reviews on whether the 2nd gen is equally as good over the gen1 on battery life.
i'd go for the ipad4 refurb. i like the Air but $150 extra is not worth it. that $150 less makes it much more likely you'll recoup a greater percentage of your initial outlay if you want to resell the ipad4 on your next upgrade.