It has been upgraded several times , but still fallout 4 uses a modified skyrim engine , which used a modified fallout 3 engine , which used a modified oblivion engine , which used a modified morrowind engine , which came in 2002 . The tech is ancient , the Gambryo engine il like 18 years old , althrough heavily modified through the years to what now is called the creation engine . While it is modified the core remains the same . If there wouldn't be any problem to just modify an engine as time goes on , why do other companies bother to build new engines from the ground up ?
Imho building a new engine costs money , but ffs they have been using the same engine since the begining of times , they should bring something new to the table .