The main problem with the newer 6004 are the quality issues. I've had one for a while I bought second hand,it was fine for a few weeks then I noticed the barrel kept creeping back and losing a good contact between the barrel o-ring and the breech face losing power to the extent the pellet wouldn't come out
I loosened the barrel clamp and tapped home the barrel to the breech face and when I tried to tighten the clamp one of the screws just kept turning without tightening up enough,the thread in the over-lever was stripped.The holes are only m3 in to aluminium,it's just not man enough for the job beyond the first assembly.Like a one use bolt.
I put it away for months before deciding to butcher it or bin it.There just isn't the room to re-tap a bigger hole,in the end I drilled right through the over-lever m4 and pushed m4 bolts through from inside and nuts on the outside.The nuts pulled the bolt heads through and pinched the barrel tight.
It looks a right state but it does work.I certainly wouldn't buy another that's for sure.