restartingrestoring my iphone 4 iPhone 4S

restoring can help all software issues. if u have software prob it is recommended to restore but if u have service issue u can go setting then reset then reset network settings.

What will restarting my network settings due? Like what will happen with my cellular data?

it will restore ur network setting only. This is done when u have network prob such as ur celular data deosn't go to 3G n remain in E. and also when u don't receive the network coverage properly.

What network settings is it deleting?

nope it won't affect anything. it is just like rebooting your network. will take few seconds.