Ah, I thought it had 2+2, meaning 2 soldered. If it has 4 soldered, adding 8 would result in 12GB. And yes, the details are sadly a secret. It is possible that if you have 4+4 right now, the replaceable 4 would give you the information you need.
For 2013, it was probably SATA3 to start with so even though SATA are backwards and forwards compatible, I think you'll end up with SATA3 drive in a SATA3 port. The speed might be a little limited by the computer (the SSD would support faster reading/writing than the port version on that specific laptop), but it'll still be faster for random access than a hard drive. In those days, a good hard drive would match or beat an SSD's speed in consecutive access (few big files). For random access, which is generally needed nowadays, SSDs still gave a big performance boost. Since then, SSDs have become a lot faster, hard drives have barely developed further.
Aside from that, I remembered a little thing that may or may not be an issue. The computer is said to have shipped with Windows 8, in which case I do not remember how good the SSD support was. It should work fine, but you may need to disable automatic scheduled defragmentation to prolong the SSD's lifespan. Basically a hard drive optimization step that would cause an SSD to read and write more than is necessary and an SSD's predicted lifespan is mainly connected to the amount of reading and writing it has to do. I remember when early SSDs became popular for users, Windows did not automatically disable it for SSDs, it was fixed at one point, but I cannot promise it was fixed for Windows 8. 10 and 11 are definitely fine. If you run Linux, any reasonably updated one should have no problems either.
