1. It is hard to hang out with friends during exam period.
Good. No change.
2. Sorry I usually make my phone mute when I sleep. That's why I couldn't notice your unanswered call.
Grammatically OK. People often say : Sorry I usually put my phone on mute when I go to sleep. That's why I didn't notice your unanswered call.
3. You can look forward to it!
Context is not clear. It's not used as a general expression of encouragement.
4. I would like to marry after I get a job.
"get married" is more normal.