engine surges at low speed and won't accelerate over 4000rpm.What do I look for?

You could have an air/fuel mixture concern, an emissions control concern, fuel delivery concern. You could also have an intake or exhaust system restriction. Have each area inspected thoroughly by a trained technician near you. There's simply no guessing that can be done without hands on inspection. A hands on diagnostic inspection should get to the root of the problem fairly quickly, so you can pin point down to the actual cause and then get the concern repaired. https://repairpal.com/repair-shops