Why is my mod_rewrite not working?

First, your additional line comes too late. The rule before catches everything you want to match.
Second, it doesn’t what you want. Your …

RewriteRule  /([0-9]+/?)$  /about/latest-news/$1     [NC,L]

… matches requests like example.com//0000000000000000000/ or example.com/about/latest-news/2010/ (infinite loop!).
The first argument for RewriteRule omits the starting /.

To match year archives you need rather:

RewriteRule  ^(\d\d\d\d)/?$  /about/latest-news/$1     [NC,L]

I’m not sure if you really need mod_rewrite for that. Try the following line above the rewrite block:

RedirectMatch Permanent ^/(\d\d\d\d)/?$ /about/latest-news/$1

You have to tell WordPress about your custom archive permalinks. You should find enough good examples under the tag.