Well yes, dig deep enough and it will be able to be capped.
I don't know why you didn't think of this when the fire was removed either - you would have saved yourself a few bob.
I have never seen X400 or X100 come in a can. Plastic bottles yes, tubes yes, cans no.
I also have to say that anyone who leaves the installation debris behind, or worse lobs it in the customers bin, is a bit of a cowboy IMHO.
Anyway, where has this slavish devition to power-flushing come...
They are not stuck on to the threads, just give the valve a good wrench - you really can't hurt anything at this stage.
It is far more likely that they will be pinching onto the copper pipe, but you won't know that until you get the valve off.
Your current terminal 4 (heat off) is not currently connected and therefore you don't need to do anything with the old 4 to the new 5.
You will need to link 2 to 4 unless you need volt-free switching (which doesn't seem likely).