Urgent! Heathrow airport terminal 1-4

Most likely you will be flying with British Airways... In this case, follow the sign 'Flight Connection' as you arrive at Terminal 1... It will bring you to the Flight Connection Hall where you will need to be taken through a security check...
After security check, you can then follow the sign for 'Terminal 4' where it will bring you to a shuttle bus stop... There will be shuttle bus bring you to Terminal 4... As you arrive at Terminal 4, you will arrive at the departure hall, where you can find the information for your connection flight...
In my experience, connection at Heathrow could be very quick during early morning... 90minutes should just be OK... But you must act fast as it is a little bit tight in time... If your first flight arrive late, the airlines will arrange for a bus to pick you up beside the plane to shuttle you to the next flight...
But beware of the problem that British Airways is not aways able to issue boarding pass for both sector at Hong Kong... Although they can send your luggage all the way to Amsterdam, but sometimes you will need to get a new boarding pass at London for the London to Amsterdam sector... In this case, just approach the airlines counter at the Flight Connection Hall where they will issue you the ticket...

Terminal 4 is 15-20 minutes away from Terminal 1 by train/coach.

1) If the connecting flight is the same airline as HK to London (e.g. no need to collect luggages)
As soon as you arrive Terminal 1, you can follow the sign to transit and they will transfer you to Terminal 4.

2) If connecting flight is different airline (e.g. need to collect luggages).
After you have collected the luggages, you will have to go through custom and then take either Heathrow express to Terminal 4. Heathrow express is free of charge. However, if you take coach or underground, there will be a fees.

90 minutes should be sufficient unless you need to check in again (via different airline).

Good Luck.