Increase ram usage?

After rebooting VPS after installation I noticed that it barely uses ram out of 4GB.

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Does this increase automaticly? It's been like this for three days, and I have a forum running on it. I want to use Memcached & Zend OP Cache..

Perhpas increase memory they use? especially OP Cache, if the limit was reached .. or even better, how to see how much opcache and memcached used? perhaps they are not even required to be allocated more ram