welcome Archie ...Essex eh ! my old stomping ground as a kid .
it might pay you to mark up your throttle into 4 segments 1/4 1/2 3/4 and WOT . I usually use a bit of masking tape and a felt tip.
Then you can test the throttle response at various parts of the throttle range and pin point which carb component and setting needs attention.
If you get a slow poor throttle response when you snap the throttle from idle to 1/2 throttle then you need to be looking at your pilot jets, air mixture screws settings and slides . You mentioned that your bike had pleated air filter pods.... well the likelyhood is that the pilot jets were increased to cope with the extra air at idle. If this is the case then you will probably need to go back down 1 stop to standard pilot jets.
You may find this jetting range chart helpful. It clearly shows how much the air mixture screw , pilot jets and slide are responsible for throttle response from idle ,right up to half throttle and beyond .
If your throttle response problem is only at around 4000rpm in every gear then mark your throttle position at that rev point and look to see which jets and components are likely to be at fault. My money is on the pilot jets, air mixture screw settings and slides .
Another test that you can do is to hold the throttle steady at 4000rpm in 4th and carefully apply a small bit of choke and see what happens . If the bike picks up it suggests that your fuel mixture is lean at that point ...if the engine bogs then you are too rich at that part of the throttle range .