I don't see why they wouldn't. They are releasing mod tools with Skyrim so its not as if they are trying to keep the engine top secret so they can reuse it and still have be an untouchable paragon of awesome.
If they don't make it, a Skyrim modder will.
NooNameLeft said:
Fallout 4 will probably be (and hopefully look) amazing but I doubt we will see it any time soon.
Unless Bethesda decide to end the series with the mmo fallout...
The MMO is being (or wanted to be) made by Interplay who are currently (this was a long time ago, but I haven't heard news of it finishing) in a legal battle with Bethesda over the rights to Fallout.
When Interplay sold the Fallout IP to Bethesda they did so on the grounds that they would still have the rights to the Fallout MMO so if one were to come along, Bethesda wouldn't be involved.