Sounds like a fuel related issue. I have had a fuel pump start to lose pressure around the 4k mark, put a fuel pressure gauge on the car and drove down the street and seen the pressure start to drop from 32psi to 20psi causing the car to bog down at 4k. Put a new fuel pump in and it took care of the problem. Check fuel pressure and see where your at... If it's the fuel pump you will see the problem on the gauge and know it's the pump since it reads pressure before it reaches the injectors. You could also use injector noid lights to diagnose bad injectors instead of swapping parts that could potentially cause another issue down the road.

Remember combustion engines need 3 things to run, air, fuel, and spark. If it's not getting one or only getting a portion of it you will experience drivability issues. So when diagnosing drivability issues those are the 3 thing you check first, after that then you start looking for the weird stuff like ECU's or wiring issues etc...