They're very common, I have no idea about the rates though. The only unbranded 4gb player that a Google search could find me was at (on the price list, in the Mp3 section, its named City MP4 4Gb, for 3,400).
I could try to find better options for you from the market itself (Uni Plaza, Techno City). Buying anything from posh areas like Clifton or Defence will cost you a few hundreds more.
Thanks for the wonderful piece of info. Would be really grateful if u could find the actual market prices.