S4 hull is under fabrication, S3 is at SBL, S2 is getting ready for sea trials. S1 is experimental sub, going on and off sea (most secretive of the lot, we dont have any info about it)
so what will be next S5.
The Admiral talked of inducting one sub every 3 years. So, if we say 2013 will be the induction date for Arihant, then we can say the next sub will come in 2016, followed by the 3rd in 2019.
S1 is a TD which was scrapped. It exists as a land based prototype. The lead sub is Arihant, as is the class name, and is the only SSBN/GN we have as of today. As you said, S-3 and S-4 will be the next 2 subs.
The S-5 will most probably be an entirely new class or even a Arihant II class. Any S-5 will come only after 2019. Since we won't know anything much about it until it is launched, we simply have to wait for the Navy or the companies to release information.
It would be interesting if a new sub development is already happening for SSNs along with a second fabrication unit and assembly line somewhere.