Given your choices, I would recommend the Kobo Touch. It has more flexibility than the Kindle 4, in my opinion. With a current firmware (you may need to manually install it, which is fairly simple and directions can be found in the Kobo forums) PDF performance is acceptable, although not great. Several people swear by koreader, so that may be something you want to pursue.
A big advantage of the Kobo, in my opinion, is that most countries' libraries use Adobe's Digital Rights Management for e-book loans. All Kobos can be used with the Adobe Digital Editions program if you have a laptop or computer available to you.
For tablets, I've had good luck with Lenovo tablets. I also use an old Blackberry Playbook as a PDF reader. It's small (7") but even after all this time the battery usually lasts me a couple of days of reading PDFs. If you can find one I'm sure it would be quite cheap.
Good luck! Let us know what you choose.
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