Manoj, Cloud, VPS, Shared hostings
All, doesnt reflect the true picture of the CPU cores allocated, as there is a hypervisor in between and depending on which hypervisor is used, they allocate CPU's to that reference, for example CPU allocation can range from 2 to 8 per core, so your cloud may have 4 cores in actual allocated to you, but each cores is assigned 4-vCPU's making it 16 core.
for php memory limit, 4GB should be more than required, have you setup memcached
server as well? in php.ini there is a setting realpath_cache, comes disabled by default, turn it and allocate default 4096MB value to see if it helps.
Besides, install phpmyadmin, and look into stats and what are the recommendations given (work bests if DB has ran for good few days).