did u cycle that new 4ft tank before you put him in? i dont see how you could transfer him with completely new water and get nitrate readings that quick, even with established filter material.
i think you should get your water tested at a LFS or just buy some tests. if the tank wasnt cycled and it's a month later, the nitrite or ammonia is probably still really high.
how hard is the water? youre taking the chlorine and chloramine out when you do water changes, right?
if its none of these things, i dnno wat to tell you. ive never had that kind of fish before and people have already suggested what he may be sick with.