Two questions:
Is it legal in competition to use a 4x4x4 as a 2x2x2 cube?
Why are transparent cubes not allowed in competitions?
It is not allowed to use transparent because you can see the stickers on the back side, which you can't do with a not transparent...
And your other question: 3h) No modifications are allowed that enhance the basic concept of a puzzle. Some examples of enhancing the basic concept are: new moves are possible, normal moves are impossible, more pieces are visible, colours on the backside of the puzzle are visible, moves are done automatically, more or other solved states.
3i) Any modifications to a puzzle that result in poor performance or results by a competitor are not grounds for additional attempts in a event