this may sound dumb or not very informative, but one obvious thing would be that 3rd density energy would not be as strong as 4d energy- to go further 4d nrg probably vibrates at a higher level than 3d nrg, simply because the 4d realm everything vibrates faster, i would assume that includes energy (nrg) ?
also, what came to mind is that a 4d entity would not have much 3d energy to spend to uphold the shapeshifting, thus limiting them for a certain duration of time- cuz he is 4d, i would also assume that he has mostly 4d energy.
i hadnt heard this explanation for why they cannot shapeshift forver, but i have heard that they cannot hold the shapeshift for too long either....
im not all too sure about all of it, to be honest. these are all just speculations on my part.
basically, a 4d entity is less physical in their make-up, this is because their atoms vibrate faster, as i was saying. considering that, you can begin to fathom why it would be hard for them to shapeshift in 3d, because 3d is very very physical whereas they aren't as solid. thus it breaks (it= shapeshift) probably not due to the 3d energy causes, but probably because their 4d energy is too "powerful" or "overrides"
sorry if im not giving you good solid factual info that is backed up.... but i hope you liked my theorizing !!
"...i was taken by the hand, from the ocean to the sand..."
nitin sawhney - 'eastern eyes'