Sometimes will throw a grenade, then when going to press '4' to ready a frag again later, a smoke grenade will have mysteriously jumped into the 4 slot from the 5 slot. No smoke will be in the 5 slot anymore. the frag will have been displaced into general inventory.

you have to remove the smoke from the 4 slot by dragging into a container (backpack, vest, etc), then CLOSE the inventory menu and reopen it before the 4 slot will accept a frag grenade again.