Why is SSJ3 shown as tiring to maintain, but SSJ4 is not? In several episodes of DB:Z and DB:GT, using SSJ3 form is shown to be very tiring for the user and burns up energy very quickly, compared to  SSJ1/2/enhanced/mystic/etc.  

However, when DB:GT rolls around, SSJ4 form, while as tiring as all other enhanced forms, doesn't seem to put as much strain on the user, nor does it have a time limit, as with SSJ3.

Is there a distinct reason for this in terms of power drain, or how the body handles the transformation, or is this perhaps something overlooked when GT was created?
Don't forget that GT is a filler, it wasn't created by the original creator, Akira Toriyama.

There is a very high probability that is was overlooked. Also if I remember correctly, SSJ3 wasn't shown as tiresome in Dragon Ball GT.