dear Dr. Shanker Lal Shrisvastava,

in my opinion the best system is the classic "cooling room" design under Controlled Atmosphere with regard to apples storage. 

To decrease the "field heat" you can consider a  precooling method (forced air or hydrocooling). You can refer to the publication of Mitchell et al., reported at this link.

Otherwise this paper:

http://www.bae.ncsu.edu/programs/extension/publicat/postharv/ag-413-1/

http://www.ba.ars.usda.gov/hb66/011precooling.pdf

Dear Shanker Lal Shrisvasrava

Please refer to the article below

http://www.fao.org/docrep/003/v3630e/v3630e08.htm

http://www.bae.ncsu.edu/programs/extension/publicat/postharv/ag-414-2/

http://postharvest.org/images/CharcoalcoolstoragePNACQ751.pdf

http://ucce.ucdavis.edu/files/datastore/234-2143.pdf

http://www.ejarr.com/Volumes/Vol1/EJBS_1_07.pdf

http://naldc.nal.usda.gov/download/IND43894022/PDF

Thanks Ahila.

forced air circulation system

If you are looking simply at air distribution with conventional DX cooling then this room is small enough to have a ceiling cooler  mounted at one end with air blown across the top of the boxes, and drawn in back underneath, ie a crcular airflow in the vertical plane.  A dual discharge cooler in the middle of the room blowing either way is better, ie 2 circular patterns, but 4m may be too low a headroom for your stacking.  Ducted discharges should not be necessary in a room this size.  The layout of the boxes should be such as to encourage airflow through them not round them.  This will depend on door position for loading / access. An alternative is a floor mounted cooler with vertical  steel spiral tube duct and a head with discharge outlets that can be angled to blow across the room.  This is generally less attractive as it takes up floor space and reduces storage volume.  Stacking abd loading are very important, as is box design.

Thanks Andrew.

Providing the food has been chilled to the required storage temperature the purpose of a cold storage chamber is to maintain the temperature around the food.  The main heat load for foods such as apples during storage is that generated by their respiration.  However this is minimal compared to the initial cooling load, so the refrigeration system capacity does not have to be as large as that required for initial cooling.

Forced air circulation systems are needed for removing initial heat, but high air velocities during storage can lead to high weight loss.  In addition, it's very difficult in practice to force air through products such as apples in bulk storage and using high velocities during storage can be very energy inefficient (see http://postharvest.tfrec.wsu.edu/pages/N1I4B).

Personally I would recommend considering a passive cooling system for long term chilled storage of products such as apples.