Not every #4 Mk2 was import marked. I have one that isn't as well. It just means it was imported before the import mark law was enacted in (I think 1968?).
The value you are seeing quoted is for a perfect still in the wrap specimen BTW. Once unwrapped you'll be lucky to get $550.00 for it.
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”