I am unaware of any efforts by them at the Federal Level to decriminalize MJ.
However if MJ is decriminalized at the Federal Level, then you can answer "No" to the 4473 question and legally by a gun.
I believe it asks "Are you addicted to .... do you partake ect.." So you already have to answer no.
But I get it, you mean "yes" and still have a valid form.