How would adding batteries make it stop working? Are they breaking a circuit? Causing a short? If two batteries were dead, would the other two still make the light work? Could you make a connection on the two spring pairs, put the two batteries in and it would also fail? Two good batteries, two low/dead ones?
Two of your working batteries and two flat top batteries…just to see if the battery body is affecting anything and assuming it wouldn’t connect to the flat tops?? Cover the other two button top batteries that you add to make it not work and see if its the battery body or the actual power they supply? That would be an interesting test and easy to try…
Is it a driver problem? Something about a chip or resistor that cuts it off for some reason? Voltage overload? some chip disregarding the additional power? Misreading the power? misregulation?
Is it mechanical or an electronic failure? Be interesting to know…
Hate to ask, but have you tried two other batteries from your collection with the two that work? Any four of them in a pair work or just a certain pair? Can you put them in any two slots or is it a certain configuration of placement/same slots? Assuming you tested any of the four in another flashlight?
When adding the other two batteries…are they causing some connection to be lost…like “pushing up or down” the head or tail…breaking the connection? Causing a spring to move ever so slightly breaking connection? Additional batteries causing some deformity in the tube or springs that cuts the power?
Like if it was any of the batteries, but in the same two working slots…something physically wrong with the other two slots?
Certainly odd…flumoxed!
not saying your dumb, just trying to say everything I can think of…