Huawei had started rolling out Android Marshmallow for Honor 4X earlier this month. The B511 OTA update is now also available for download and can be installed manually.
At present the update is being rolled out to the Indian variant of the Honor 4X Che1-L04 and weighs in at 1.18GB with build number Che1-L04C675B511.
The EM 4.0 (Marshmallow) update is available over-the-air and if you still haven't received it, you can check for updates from Settings. If you can be a bit more adventurous, you can download the OTA update file from here and install it from recovery. You can find the steps from the source link.
The Honor 4X was launched in 2014 running on Android KitKat, with Snapdragon 410, 2GB RAM, 8GB storage.