Are you looking for a 3.5" (sometimes referred to as desktop drive) or the smaller 2.5" that use the standard laptop drives?

 

The reasons I am asking is that 4TB drives are pretty new, and while you will be able to get a 3.5" drive, you will not easily find the 2.5", and if you find, the price will not be justifiable above what you would normally pay for a 3TB drive.