For DDOS versions, you can check https://support.emc.com/kb/184446. For upgrade path, you can check release notes. Rule of thumb is that you can jump at max with 2 major releases (where 5.3=5.4 so you can go from 5.1.x to 5.4.x). I'm not sure why you are interested in 4.9.x and 5.0.x if you installed it last year, but you can find pretty much everything using search.
4.9 and 5.0 are long since End of product support.
5.0 was eops 31st January 2014
5.1 was eops 30th September 2014.
I can't seem to find 4.9 listed but I believe it was about September 2013.
Although I agree that the old DD website was better for DD specifically, support.emc.com still provides the same content.
Many customers wanted a single portal for all their EMC products and so this was always going to happen, luckily it took a long time to allow us all to get used to it, both existing Data Domain customers and new EMC customers.
On the main landing page of support.emc.com I searched;
"4.9 command reference" without stating a product name (as "data domain" for example).
I found the administration and command reference guide for 4.9. Also all the release notes are listed for the minor releases at 4.9.
Although a DDOS is EOPS, EMC will not force you to upgrade DDOS unless there are specific issues that you have and we have fixed in that time, 5.0 first went GA on 27th January 2011 - that's a pretty long time to be fair and so the fix list will be huge.
We just won't be able to help you further if you have a problem and don't want to upgrade to a fixed code release.
I believe your concerns and queries are answered here, but if you have further concerns or I missed something, please let us know.