Oh, as for the advice regarding hard drives, is there a good reason to replace the hard drive with a SSD? My current hard drive seems to be working well. If it were to give out, I would definitely consider replacing it with a SSD. Basically, what I need to know is would not having a SSD impact the performance on Sierra greatly?
I definitely want to keep the CD drive, as I actually still use it to import my music CDs onto iTunes, as well as to play really old computer games.
I suspect that those 4GB Macs you were looking at had an SSD for storage, so that when they did need to page and swap data to disk, it was very VERY FAST, so you do not notice the lack of RAM nearly as much.
The speed accessing an SSD vs a rotating hard disk is orders of magnitude faster.
Also, while your hard disk may appear to be doing fine, it is also possible, due to its age, that it is starting to do more silent retries when asked to read data. It is a mechanical device and subject to wear and tear.
You do not need to do an SSD at this time, but if you find that Sierra feels really slow compared to Snow Leopard, and you have removed all the unnecessary 3rd party additions you have accumulated over the years (EtreCheck can help with that <http://etrecheck.com>), and you want to keep that Mac, then consider an SSD upgrade.
As for the DVD, you can get an external enclosure for the DVD and connect it via USB for the few times you need to import CDs. That is what I did with my iMac's DVD.
I am very serious that when I went from Snow Leopard to El Capitan last Christmas, the SSD made my iMac run faster than when it was on Snow Leopard. I was already at 16GB so the RAM was a constant for me. About 6 months later I replaced the disk in my Macbook Pro with an SSD, and again, it is faster than when it was running on the original operating system it shipped with (Lion), but now it is running El Capitan.