Well what else are they working on? They are almost done with Dungeon Siege III (which comes out in June), so I'm pretty sure they are going to have time to make more dlc after Lonesome Road.
Very doubtful. You don't let a bunch of people go if you are going to need them later. Obsidian has other projects, of course, but obviously not enough to keep those who were working on the New Vegas DLCs going when future DLCs beyond the four announced are very unlikely. There's a reason why the summary for Lonesome Road includes the term "full circle."
If the DLCs did monstrously well, then maybe we'd see more. But Bethesda's next big game, Skyrim, is out in November so they are going to want the focus to be on that game and its DLCs, not more stuff for New Vegas (even if it is still popular, being in the top 20 of games played on both XBLive and Steam in recent weeks.)