This has become more of a turbo boost questions and might be good to use the search function or just repost. I'm not great with the turbo operation, but seems like the either the wastgate lever is not operating propertly or not opening all the way as it should.

Exhaust flow should propel the turbo (especially at 4000 rpm) and create plenty of boost. Have you checked the play on the turbine from the front for wear?

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1987 300TD 147,000 miles- Palomino leather interior, 1995 facelift and body cladding, E350 wheels, Rebuilt suspension and sport springs, rebuilt turbo, New Monarch injection pump and injectors....and the list goes on and on...