UPDATE :
after using "http://172.18.4.5/ocsinventory" as url for the agent on the windows pc , even if i get an error #404 in the agent logs i get this in /var/log/apache2/access.log :
172.18.4.57 - - [03/Feb/2017:16:05:00 +0100] "GET /ocsinventory/deploy/label HTTP/1.1" 404 460 "-""OCS-NG_WINDOWS_AGENT_v2.3.0.0"
172.18.4.57 - - [03/Feb/2017:16:05:01 +0100] "POST /ocsinventory HTTP/1.1" 404 447 "-""OCS-NG_WINDOWS_AGENT_v2.3.0.0"
172.18.4.57 - - [03/Feb/2017:16:05:11 +0100] "GET /ocsinventory/deploy/label HTTP/1.1" 404 460 "-""OCS-NG_WINDOWS_AGENT_v2.3.0.0"
172.18.4.57 - - [03/Feb/2017:16:05:11 +0100] "POST /ocsinventory HTTP/1.1" 404 447 "-""OCS-NG_WINDOWS_AGENT_v2.3.0.0"
So the agent successfuly communicates with apache and the server pc but i still get nothing on OCS