Please suggest bookshelf with best bass response under $4000 pr.

Acoustic Zen adagio junior or Bryston Mini T, They are substantial big for bookshelf speakers but will bring you deep strong bass  as stand floor speakers .

There are so many really amazing bookshelf speakers e.g. Kef, BW and so on.  I would try to listen to the newer Elac bookshelf speakers designed by Andrew Jones.  You could save the extra money for a subwoofer (again, so many great designs) to take care of those frequencies that you can really feel.  Again, check out Elacs sub offeringa as they are so cost-effective.
Lastly, matching your amp to the speakers is critical and there isn't a better method than listening to the setup.  If you can find the right dealer they will accommodate you.  When I purchased my last speakers I carried my (heavy) tube amp to two dealers who wired it up for me to really get the sound I was listening for.  Take your time...  and enjoy!

Bass response is so much more than the speakers themselves. Placement relative to listening position, and room relative to same are critical elements. Getting the bass "right" at the MLP, is in my opinion the most significant thing in this hobby. It is very much worth the effort to learn how to accomplish this.
To that end I'd be of them mind to add a sub with EQ to your existing setup. But really you should measure what's going on at your listening position so you know where to focus your attentions. A Umik1 & REW would get you there.. 
Others are right about the limitations of driver vs cabinet size etc.. but as mentioned above there are speakers like Golden Ear's Aon that use passive radiators to effectively increase cabinet size and bass response. 
One thing a lot of audiophiles either forget, or don't know is that as the frequencies get lower, our ability to hear distortion decreases proportionately and also we perceive them to be quieter relative to the rest of the spectrum. (Search Munson curve)
If a speaker like the Aon is rated +/- 3db from 38hz & up it doesn't mean there isn't effective bass below that frequency, and you could probably not physically hear between 5% to 10% distortion down there anyways. The room itself can create as much as 30db swings through the audible range and usually the biggest swings are in the bass frequencies. 
IMO getting the bass right creates the clean canvas for the rest of the spectrum to paint on. 

Dynaudio C1 wonderful bass !!!!

Dunaudio C1. Just wonderful. Used for $4 or less. Be aware there are a couple iterations.