While you are in the UK you must be very careful not to break the conditions of your Tier 4 visa.
If you breach your Tier 4 visa, for example by breaking the rules of how much you can work as a student on a Tier 4 visa, you may not be allowed to stay in the UK. All the conditions of your Tier 4 visa will have been given to you when you received it. Any special conditions will be on your Tier 4 visa or Biometric Resident Permit.
Tier 4 visa conditions are very strict, and if your circumstances change your Tier 4 visa may become invalid. The Home Office website has full information on the visa implications of changes to your circumstances.