A5/X7/BT4 markers all use the same threaded barrels, so if you already have a barrel for the X7, it will work on the BT4. I'm currently using my X7 12" Lapco barrel on my BT Delta Elite for example.
Shrouds are a totally different story unless they mount to a Picatinny Rail, they're usually specifically designed for a certain marker. I very much doubt you could get an A5 shroud to fit on a BT4 unless you performed some adjustments to it (with a Dremel or Hacksaw)
Those X7 Shrouds you linked to are designed to specifically fit the X7, so would not work on the BT4 (I have the SD Shroud on one of my X7's).
I believe I'm correct in stating that the 98 Stocks should fit the BT4 but having not actually tried it, I can't guarantee it.