i would make sure that your converter is ok first as with only 4000 miles on it, it should last way longer then that even with alot of power in front of it unless some one didn't do it right but have someone check your converter first and check your transfilter as it could just be pluged with band shavings from being just rebuilt

 

for only having 4000 on it im sure its your filter or even the converter but lets hope its your filter , just drop the pan and see what all is in the bottom of the pan and see if its full of shavings if so just clean it out good and change the filter , usually after you get a trans rebuilt you should always change the filter about every 2000 for the first time cause the shoes are like brake pads , they wear till they get conditioned and the shavings have to go some where but not everyone does it and they end up burning the new trans out