Richard, exactly what I was just researching, and had thought perhaps that suggestion might be a bit tenuous, so Im glad I was not alone!!
The East India Company of course had the heart and VEIC letters quartered with a cross atop. This was of course in line with the well known globe and cross used in marking arms and many items with its Christian connotations.
With the EIC the mark with the cross was intended as a balemark to identify and as a kind of protective amulet mark.
With trade to India, it has often been claimed that the cross was altered by adding a line, into a four, to avoid offending or concerning the Muslim trade partners.
These EIC markings were of course well known throughout India, and in the Anglo-Afghan war of 1839 the markings became prevalent from the locks of the Brown Bess muskets captured by Afghans. The use of these EIC hearts which were topped with a '4' became popular with Afghans using the locks from these guns through the century to fashion their own jezails.
Tribal people tend to see such images symbolically rather than of course their original meaning in the context of origin, and may have seen the '4' as a mark of quality or protective imbuement. They likely disregarded the EIC heart as of course an element of British aggression, much in the way Tipu defaced the mark on his cannons adding his own over them.
Purely speculative, but as Richard has observed, a reasonably plausible idea.