One More Week of Explorer's Ring Event

I'm talking in terms of The quests level limit. Like is the quest only available at level 40 and at 41 its gone ?

No, it should be just fine. I got to level 40 while in white mamas and kept going to 41 while in there. I was able to turn in the level 40 quest and then accept the ring (level 50) quest. As long as you're inbetween 40 and 49 and finished the previous quests, I think you should just be able to pick the quest back up, assuming you don't get to 50 before the event comes back.