I'm not sure these numbers mean much. My 1st gen Echo is 4620, 2nd gen ones 8120 and my 2nd gen Dot is 8320. You can't force updates on these, but on 3rd gen devices you can ask Alexa if her software is up to date.
I've given up on having my 2nd gen Dot and Echo in multiroom; whatever I do one of them is unreachable now.