Not very clear from your post, but I believe you ordered 4x Nexus. In which case the customs officer decided (and he's fully justified, IMO) that these are not for personal use and meant for resale. Which makes you a commercial importer.
So GoI has this rule where certain types of electronic goods, if imported for commercial purposes, require the importer to have a BIS (sort of ISI quality) certificate that, well, certifies that the said goods are not emitting harmful radiations, electrically hazardous or suchlike; and therefore safe to use. To be able to certify, the importer should have a testing lab and whatnot. Big monies needed.
If you've indeed ordered 4x, then this is what has happened. The officer is following the rule book - thinks you're an importer of tablets, and so wants the BIS certificate. I also don't think just any Nexus BIS certificate will do. It should be *your company's*.
I really have no idea how you can get out of this. Talk to some customs clearing agent in Kolkata who has an office in Delhi. Maybe they'll know what to do.
Additionally, if you want this delivered to Kolkata from Delhi, you'll need something called a waybill. That's another hassle.