I posted something similar like 6 months ago with the KFL+ and Fogger V3. I bought both and a few drip attys and only use the kayfun from that batch of purchases (I use different rdas, but none of the crappy ones I first bought). With that (and knowing little about the v4) I'd say go with kayfun light +.. its tried and true.
Eciggity.com has the KFL+ V2 clone
here for $35 with a drilled out air hole. If I had to replace mine, I'd probably go that route.
here is all of their kayfun stuff.
If your just looking for a better vape and dont want all the 'hastle' of the RTA's you could try the Aspire Natalus Mini (
here) with the BVC coils. I haven't tried them but all the reviewers I watch said its up there with the kayfun.
(I enjoy rebuilding, but some claim its too much like work.)