I still have a decent supply of 4320, partly because I try to keep at least some of most rifle powders on hand (you don't want to know how many). It's a good powder, but when powder companies discontinue one it's almost always due to one reason: It's not selling well enough for the manufacturer to spend the time change over their production line to make more.
This can be blamed on "too many" newer powders, but despite what many older handloaders prefer to believe, many of the newer powders are indeed superior, often in more than one way. Which is why they sell better--especially among target shooters, who burn far more powder than the average plinker/varmint shooter/big game hunter.
One additional comment: I have a big collection of old gun magazines and books, and believe it or not some contain complaints about how popular powders were being discontinued--due to new-fangled powders like the IMR powders DuPont in the 1930s--which included 4320.
I suspect IMR3031 might be discontinued the future, but it has a burn-rate that's not as "duplicated" by newer powders.
