I was interested in buying a TI-4 Plus CE (you could probably tell by the title ), and I have a few questions to ask before making a purchase:

1. How can I tell if it has the latest OS?
When I buy the calc, I really want to make sure it does not have OS 5.5.1. I don't know if there are any tell-tale ways to find out the OS version while online shopping, but if there are please let me know!

2. Does Doors CE 9 exist? If not, what other shells are there for it?
I have been wondering about this question for awhile, as I have seen people's signatures on Cemetech say Doors CE 9 user, but have never found an official release.

3. Does anyone have a recommendation on a specific one I should buy?
Basically, if you find a TI-84 Plus CE, that you think looks good, and is not way too expensive, post the link so I can check it out!

EDIT: Oops, sorry, I should have put this in the "Calculator Help" section, not "Calculator Hardware, Electronics, Robotics". Is there a way to fix that?

1. OS 5.3.1 isn't so bad since you can downgrade to OS 5.3.0. It's OS 5.5.1 and higher you don't want. Any used calculator at this point won't have ASM removed yet. If you're really set on getting an old calculator, you can ask the seller what the date code on the back of the calculator is. If they tell you something like L-0315 then you know it was released in 2015. Since OS 5.3.1 was released in 2017, any calculator with 15 or 16 at the end will definitely have an older OS.

2. No, Doors CE will likely never exist due to Kerm's lack of time and ASM being removed.

3. Get a revision M or N calculator. Again, can ask the seller what the date code on the back of the calculator is and if they tell you something like L-0519M then you know it's a revision M calculator. Revision M is typically 3x-4x faster than the old calculators.

Does this one look good?

In addition to what TheLastMillennial said, only CEs with the newer blue packaging will be revision M or higher. I personally have a TI-83 Premium CE Edition Python, which has Python if you're into that. An edition Python is also guaranteed to be at least rev M, although there is technically a slight risk it will come with OS 5.5.1 (chances are it will still be on an older version, though). When I bought mine from amazon.fr, it was cheaper than the TI-84 Plus CE. There is also the 84+CE-T Python Edition, but that will come with 5.5.5.

The one you linked has the blue packaging (and the new serial number), so it should be good.

Thanks for all your help! I purchased it today and it will come by August 22.