There are still places which have some FS4 add-ons in stock. Computermate
Supplies in the UK sends out regular lists (and I've bought stuff from them in
the past), and their latest list includes several FS4 items.
Including the Tahiti and Hawaii scenery disks, Sound and Graphics upgrade, and
the old Scenery Enhancement Editor.
You don't need any add-ons for Scenery Disks. They pre-date the scenery
designer programs. The later Mallard scenery (which I think these may be) was
pre-converted to run from a hard disk, in the same format as the scenery file
included with the program.
For anyone interested, e-mail is firstname.lastname@example.org, or
Hope this helps.