Flat spot at 4000rpm

[ESDavey]

I was at Silverstone yesterday and noticed that I have a slight flat spot at 4000rpm. When under hard acceleration, its as if there is a pause at 4000rpm then the engine kicks in again pulling hard upto 5100. When on the dyno (prior to a new standard cam fitted in May) I had peak torque at 4100 but peak BHP at 5600.

Any suggestions as to the cause of this flat spot ?