4g on 2100MHz or band 1 is possible in coming days. First 3g auction on 2100MHz is non-liberalized, so operators can't launch 4g on that. But the recent 2100MHz auction in mar2015 is liberalized spectrum and can be used to launch 4g. For eg in tn Airtel has got 2 x 5 MHz of band 1 spectrum [of which 1 x 5MHz is liberalized]. They can either launch 3g on 10MHz of band 1 or 3g on 5 MHz and 4g on 5 MHz of the same spectrum...
Further in 2016, 15MHz of liberalized spectrum on band 1 [2100MHz] in each circle [20MHz in case of Delhi, Mumbai and AP circles] is coming for auction.
Reg 900 MHz, it's more used for 3g and though it can be used for 4g, that's not matured enough.
Reg band 20, it's used in Europe and not in India.. Indian 800MHz doesn't comes under band 20..