The cheapest 4 gb pen drives are available for 450-500. But I won't suggest you to go for them. Go for Sandisk Cruzer Contour or Corsair Voyager series. I will recommend Corsair more. I am using it for the past few months and it has really fast performance.
Are you going to use it casually or store 'critical' information on it?
If you will be storing critical data on it, I suggest you buy one with advanced protection such as fingerprint protection! I have one fingerprint protected pendrive from Adata. Ofcourse it costly, Rs. 650/- for 1 GB, but its worth for the peace of mind it gives me when I am traveling in crowded trains and buses. Earlier, I used to check every 5 mins if the drive is there in my pocket or not! Besides, its a cool thing to impress your friends