Upgrading from OpenGL 3.2

Most models in OpenBVE are not made by 3D modelling software, but rather written code in software such as notepad or notepad++. This means that most models are loaded in faces that exceed 3 vertexes most of the time, Only OpenGL builds before 3.3 support quads+ but when objects are parsed they are converted into tris anyways, so why does the sim not use 4.6 instead?

The short answer is that it'll do no good (at least at present).

Upgrading the OpenGL version in use doesn't have any magical speed effects, unless you re-write the existing code to take advantage of any new features.
It's taken the better part of two years to get a 100% GL3 renderer working, and the kinks worked out, and I can't see any immediate advantages to shifting to GL4+