I swore I wouldn't post out here again, but I find some of the comments here and elsewhere regarding BGS's graphics quality to be inaccurate or outright false. I guess people have very short memories.
Morrowind was one of the most demanding games graphically when it was released in 2002.
Four years later, Oblivion was one of the most demanding games graphically and had one of the highest system specs of any game on shelves. Oblivion and Crysis were two of a handful of games that were typically used and cited for benchmarking PC system performance. In fact, Oblivion was one of the primary games (along with Crysis and a few others) that Nvidia cited as good titles for using their (then recently introduced) SLI graphic card technology (i.e., joining two or more cards together to improve performance for graphically-intensive games).
FO3 was based on Oblivion's engine and had similar specs when it came out in 2008, so it was also fairly high tier, although tech on PC had progressed in the two years since Oblivion.
Skyrim was improved from FO3's engine and was the first game after Bethesda purchased the NetImmerse source code after Emergent went bankrupt. NetImmerse was the original name given to the engine that later became GameBryo and then became Bethesda's Creation Engine (for Skyrim).
Just as another reminder, Gamebryo was also used for another extremely demanding game at the time when Oblivion was being made: Civilization IV used Gamebryo.
The graphics for FO4 in the trailer are very good and certainly top tier, quite similar to the level of Oblivion versus Crysis in 2006.
Keep in mind that the first (beginning) and third (ending) of the trailer are cinematic, but the second (middle) section is in-game. Todd has stated (and Pete Hines has affirmed) that BGS's policy is to not use purely cinematic trailers because they feel that doing so is misleading (to put it nicely). Todd insists (and Pete supports) waiting until they can offer a trailer depicting actual in-game quality. Not for teasers like Skyrim's (30 seconds or so), but for actual, full trailers (a few minutes).
Finally, people who think another engine can do what Bethesda's does need to offer some examples of games with thousands of interactive objects throughout the game world. I have yet to see another company offer such a game, and certainly have not seen an engine that can do such work, not in the West or in East Asia. There may be one, but people who complain have never, to my knowledge, offered such examples when they make comments.
Everything I have posted here can be checked by searching for articles from the various time periods of the respective games.