"Maintenance" is perhaps a bit of an unfortunate choice of words, because which company is it again who has so much appreciation (/sarcasm) by it's users because of it's "maintenance plan" where you end up paying each (half) year?
Fortunately in this case it's simpler, because if I understood correctly from Oshyan it basically means: buy TG3 now and get the first TG4 release guaranteed.
People who bought TG3 before probably have to pay for the update.
Since it's a whole version update and worthwhile (I guess a couple here know how I felt about TG2 vs TG3) to update it is justified imho.
I hope that somewhere in Q1 2016 we testers are allowed to showcase more TG4 renders and perhaps render statistics.