Where did you read it will come?
If it's about the Jailbreak announced by KaKaRoTo, then he said the flaw to this jailbreak is still present in firmware 4.0
So you can update, it will be possible on both 3.7x and 4.0
If you read another cfw info, check if it's only for 3.7x
if it's the "JFW DH" (latest cfw release), I didn't read anything about it yet. (but I think it's only for 3.41 and 3.55)