The exploded view of the Beolab 4000 speaker is available on BEOWORLD in the service manual section. This alone is invaluable to your project: With an upgrade to Gold membership all of the product technical manuals are available to you.
The top and bottom gasket is specific to the installation, and is needed to do the job right. As a few of us have found out, and you now know, this isn't the easiest of projects. In fact, it can be downright scary ! ( Especially with my wife standing over me working, silently with her arms folded ).
As mentioned above, the use of a hot electical knife with variable temperature setting was best in removing my originals ( Gold to Blue by the way ). Too hot and the alu is scorched, so use the lowest possible setting. Pad and protect your work along the way, even if you do not wish to save the top and bottom of both speakers.