Fallout 4 will probably use RAGE or the ES5 engine
Id Tech 5-highly unlikely...maybe next gen
Rage will probably release sometime next year and it's a safe bet that neither ES5 nor Fallout 4 will make it out anytime this year, so I don't think it's out side the realm of possibility that id Tech 5 will be used, especially since id's been under the Zenimax banner for a full year already.
My reasons are more engine related
-Main reason why RAGE runs so fast is baking everything possible into megatextures,but obviously Bethesda needs 30 fps...so that would work well
-then you have how easy(cheap) is the engine for building Bethesda like RPG,people have to be trained,etc
-megatexture size is also the issue...multiple 360 discs ahoy
And there is one more issue
If those games are going to be released anytime soon their development started ages ago...Bethesda is not going to switch engines just like that...unless Gamebyro has spontaneously self-combusted
This is from one year old interview
GameObserver: After Oblivion, what made you guys decide to reuse the Gamebryo engine for Fallout 3?
Ashley Cheng: Gamebryo allowed us to get up and running very quickly with Morrowind. Since then, we’ve taken this core technology and added new features on top of it. We’ve actually had the guys at Gamebryo come to us, and ask if we would be interested in a particular shader or feature, only to turn them down and say, sorry, but we’ve already written our own version. With each new project, we scope how much time we have and decide which systems to upgrade. A major reason many projects are delayed or never ship is because the developers decide to start from scratch. Games these days cost way too much for that kind of reboot. For most programmers, it’s easier to write new code than to read old code -- it takes a lot of discipline to selectively upgrade parts of your technology and to maintain others.
GameObserver: How does the Gamebryo engine differ from other engines, like Unreal, given the types of games you tend to make?
Ashley Cheng: We’ve been building on top of Gamebryo for the past 8 years. There is no replacement for the learning and iteration that goes into building technology for that long. An engine is simply a tool. Anybody can use a paint brush and paper – it’s how you use it to create your art. It is the same with game engines.
We are very particular with our needs and they are mostly related to our editor. Our editor is powerful, allowing our content folks to quickly create quests, conversations, cities, dungeons, landscape, etc... you name it -- we do it faster than anybody else in the industry. No one can match us when it comes to vast, open-ended beautiful worlds full of NPCs, quests and dialogue.
GameObserver: Are you planning on changing or updating the engine in future projects, and if so, what changes are we to see in future games from a technical point of view.
Ashley Cheng: Absolutely. While we would never start from scratch completely, we often redesign and upgrade systems between projects. The important thing is to properly scope your time, resources and priorities before you start the work. We also spend a lot of time in preproduction, trying new things out -- this is one of the most important phases of our development.
GameObserver: What limits did you have while using Gamebryo and what do you wish the engine was capable of doing in the future?
Ashley Cheng: We’re pretty happy with Gamebryo -- because of all the years we’ve had iterating and updating our technology, we’re at the point now that we decide our own destiny. We definitely evaluate new stuff coming in future versions of Gamebryo and are always looking at what the best options are.
