Many of my No4's come to me as ex-range rifles (Fullbore as it is known here in Australia) They are predominately good shooters but nearly always total "mix masters" , as compared to those No4's ive got that were not once range rifles - it means little to nothing to me - im a shooter not a "marks and stamping's" fixated collector.
My 2 most accurate No4's both started as Matby made, but one know wears savage wood, and LB bolt head. The other has BSA bolt head, what i believe to be LB wood, Faz rear sight and Savage mag.
If indeed your foresight is a Tunnel sight as i would call it (or "globe sight" as i gather they are referred to in US and elsewhere) and you are adamant you want to change it out, there will be those who will be willing to part with some $ for your "Globe sight"