Bethesda are not dropping the ES series. In fact, many of us believe that they are working on ESV now.
We base this on....
Obsidian developing New Vegas (which is not Fallout 4), freeing Beth up for other projects.
The have apparently been working on their current 'unknown' project since before Fallout 3s release.
They aren't going to release Fallout 4 so soon after New Vegas.
Various trademarks which have been registered.
The fact that Bethesda isn't going to abandon their signiture series.
This is better continued over here in this thread ----> http://www.gamesas.com/index.php?/topic/1106754-official-tes-v-speculation-thread-55/