Hello Prem,
These functions were moved from 'nathelper' to a separate 'rtpproxy' module, but otherwise largely unchanged in nomenclature.
http://kamailio.org/docs/modules/4.0.x/modules/rtpproxy.html
The 'rtpproxy_sock' parameter is still there:
http://kamailio.org/docs/modules/4.0.x/modules/rtpproxy.html#idp16978832
-- Alex