I have a Mazda single cab automatic short bed 2wd 2.5l b2500 with 260,000 miles that runs great.
There is only one problem with it. When im in Park or Neutral and if I rev it up between 2000-3500rpm and quickly let off it will fall to 350 or so rpms and shake the whole truck because its running so rough, If I get it over 4000rpm it will simply come down and die.
I have replaced like 4/8 spark plugs and the TPS sensor, the IAC sensor, Fuel filter, air filter, oil...It is not pulling any codes except saying there is a misfire detected on CYL 4 and The O2 sensor is bad, the rear one thats behind the cat.

What should I do from here?

Thanks

 

I'm no mechanic BUT several forums and guys who turn wrenches suggested I change ALL the plugs and wires on my 99 B2500. I had similar problems.
What a difference!

From your post is looks like you said you changed 4 of the 8 plugs? If so, you need to do all of them. If that doesn't help I wonder if there's a vacuum leak somewhere?