I can't remember about Glock magazines - is it that older Glock mags won't work in Gen 4s? But Gen 4 mags will work in older Glocks?

I know the new model Glock magazines will work in the older model pistols. Why don't you call Glock to see if the reverse is true?

I went to the Glock Forum - apparently the different generations of magazines work in the different generations of Glocks, just that you have to use a Gen4 mag in a Gen4 pistol if you swap the magazine release to be left handed.

Gen4 mag in Gen3 Glock